Induction of beta-lactamase production in Aeromonas hydrophila is responsive to beta-lactam-mediated changes in peptidoglycan composition

Genotyping and antimicrobial susceptibility of Aeromonas veronii isolated from aquatic animals in Sichuan, China

As one of the motile species of Aeromonas, Aeromonas veronii is widely distributed in aquatic ecosystems and has been determined as an important pathogen for aquatic in addition to terrestrial animals and humans. In this study, we collected 34 A. veronii isolates from aquatic animals in Sichuan, China. Six housekeeping genes (gyrB, groL, gltA, metG, ppsA and recA) were used for multilocus sequence typing. The results showed that the isolates had high diversity, of which 21 subtypes could be divided into two groups. The group B consisted of 15 new sequence types (ST861-875), and the group A was more closely related to strains from water. Besides, the antibiotic susceptibility of each isolate was determined using the disc diffusion method. Discs of 17 antimicrobial agents were used, including penicillin, cefaclor, cefotaxime, meropenem, amoxicillin, kanamycin, gentamycin, amikacin, neomycin, ciprofloxacin, enrofloxacin, tetracycline, doxycycline, florfenicol, cotrimoxazole, rifampicin, and azithromycin. Most isolates showed resistance to 17 antibiotics such as penicillin, enrofloxacin, and doxycycline. Moreover, high-throughput qPCR was used to detect ARGs of the isolates. A total of 62 ARGs were detected positively based on 95 ARGs detection, in which class aminoglycosides, beta-lactams, and multiple drugs were the most prevalent. Our study indicated that different genotypes showed different distribution of ARPs and ARGs. To our knowledge, this is the first report of molecular subtyping and antibiotic resistance profiles in A. veronii from aquatic animals in Sichuan, these data provide a valuable referee for the prevention and control of corresponding diseases.

Multiplex PCR assay to identify clinically important Aeromonas species

The genus Aeromonas is increasingly implicated in human infections. However, accurate species-level identification remains challenging, particularly in clinical microbiology laboratories. This study aimed to develop a multiplex polymerase chain reaction (PCR) assay to identify four Aeromonas species-Aeromonas hydrophila, Aeromonas caviae, Aeromonas veronii, and Aeromonas dhakensis-most frequently associated with human infectious diseases. A total of 788 whole genome sequencing (WGS) data sets from 31 Aeromonas species were analyzed to identify open reading frames (ORFs) specifically present in A. hydrophila, A. caviae, A. veronii, and A. dhakensis. Primer sets were designed based on sequences of ORFs specific to each species to develop a multiplex PCR assay. To validate the efficacy of the assay, 256 clinical Aeromonas isolates were tested, and the results were compared with taxonomic affiliation inferred by WGS data, along with 19 type strains. The multiplex PCR successfully identified all strains of the four target species and produced no amplification in non-target species strains except the band for internal control. The multiplex PCR enables rapid and reliable identification of four Aeromonas spp. commonly involved in human infectious diseases.IMPORTANCEThe multiplex PCR assay facilitates accurate identification of clinically important Aeromonas spp. in clinical microbiology laboratories, providing crucial information to guide appropriate antimicrobial therapy and advance understanding of the epidemiology of Aeromonas spp.

Aeromonas Species Diversity, Virulence Characteristics, and Antimicrobial Susceptibility Patterns in Village Freshwater Aquaculture Ponds in North India

BACKGROUND/OBJECTIVES

Motile aeromonads are ubiquitous aquatic Gram-negative opportunistic pathogens with environmental, animal, aquatic, and human health implications.

METHODS

Motile aeromonads were isolated from village pond water samples (n = 100) of the Hisar district of Haryana state in India. Selective isolation and enumeration were followed by biochemical and genotypic identification using gyrB gene; evaluation of seven putative virulence factors and antimicrobial resistance studies and determination of extended spectrum beta lactamase (ESBL) and AmpC beta lactamase (ACBL) enzyme-producing abilities took place.

RESULTS

The viable counts of motile aeromonads varied from 1.6 × 102 CFU/mL to 1.2 × 108 CFU/mL. Six species of Aeromonas were identified with high prevalence of A. veronii (74.7%), followed by A. caviae (8.9%), A. hydrophila (7.6), A. jandaei (5%), A. sobria (2.5%), and A. dhakensis (1.3%). PCR amplification of seven genes related to virulence indicated that the majority of the isolates were positive for enolase (eno, 98%), cytotoxic enterotoxin (act, 88%), and hemolysin (asa1, 86%). Many isolates were also positive for type III secretion system inner membrane component (ascV, 53%), ADP-ribosylating toxin (aexT, 47%), and extracellular hemolysin (ahh1, 4%). The antimicrobial resistance (AMR) profile of the isolated Aeromonas isolates indicated the high resistance observed to nalidixic acid (40.2%), cefoxitin (33%), and imipenem (6.2%). In addition, the occurrence of 10.3% ESBL, 32% ACBL, and 29.9% multi-drug resistant (MDR) isolates is alarming. Phylogenetic analysis of gyrB sequences of A. veronii isolates (n = 59) together with GenBank sequences of A. veronii from different geographical regions of the world indicated high genotypic diversity.

CONCLUSIONS

the village aquaculture ponds in Hisar district have a high occurrence of MDR A. veronii, A. hydrophila, and A. caviae, posing significant animal and public health concern.

Cell envelope structural and functional contributions to antibiotic resistance in Burkholderia cenocepacia

Antibiotic activity is limited by the physical construction of the Gram-negative cell envelope. Species of the Burkholderia cepacia complex (Bcc) are known as intrinsically multidrug-resistant opportunistic pathogens with low permeability cell envelopes. Here, we re-examined a previously performed chemical-genetic screen of barcoded transposon mutants in B. cenocepacia K56-2, focusing on cell envelope structural and functional processes. We identified structures mechanistically important for resistance to singular and multiple antibiotic classes. For example, susceptibility to novobiocin, avibactam, and the LpxC inhibitor, PF-04753299, was linked to the BpeAB-OprB efflux pump, suggesting these drugs are substrates for this pump in B. cenocepacia. Defects in peptidoglycan precursor synthesis specifically increased susceptibility to cycloserine and revealed a new putative amino acid racemase, while defects in divisome accessory proteins increased susceptibility to multiple β-lactams. Additionally, disruption of the periplasmic disulfide bond formation system caused pleiotropic defects on outer membrane integrity and β-lactamase activity. Our findings highlight the layering of resistance mechanisms in the structure and function of the cell envelope. Consequently, we point out processes that can be targeted for developing antibiotic potentiators.IMPORTANCEThe Gram-negative cell envelope is a double-layered physical barrier that protects cells from extracellular stressors, such as antibiotics. The Burkholderia cell envelope is known to contain additional modifications that reduce permeability. We investigated Burkholderia cell envelope factors contributing to antibiotic resistance from a genome-wide view by re-examining data from a transposon mutant library exposed to an antibiotic panel. We identified susceptible phenotypes for defects in structures and functions in the outer membrane, periplasm, and cytoplasm. Overall, we show that resistance linked to the cell envelope is multifaceted and provides new targets for the development of antibiotic potentiators.

Phenotypic, molecular detection, and Antibiotic Resistance Profile (MDR and XDR) of Aeromonas hydrophila isolated from Farmed Tilapia zillii and Mugil cephalus

In the present study, Aeromonas hydrophila was isolated from Tilapia zillii and Mugil cephalus samples collected during different seasons from various Suez Canal areas in Egypt. The prevalence of A. hydrophila, virulence genes, and antibiotic resistance profile of the isolates to the commonly used antibiotics in aquaculture were investigated to identify multiple drug resistance (MDR) and extensive drug-resistant (XDR) strains. In addition, a pathogenicity test was conducted using A. hydrophila, which was isolated and selected based on the prevalence of virulence and resistance genes, and morbidity of natural infected fish. The results revealed that A. hydrophila was isolated from 38 of the 120 collected fish samples (31.6%) and confirmed phenotypically and biochemically. Several virulence genes were detected in retrieved A. hydrophila isolates, including aerolysin aerA (57.9%), ser (28.9%), alt (26.3%), ast (13.1%), act (7.9%), hlyA (7.9%), and nuc (18.4%). Detection of antibiotic-resistant genes revealed that all isolates were positive for blapse1 (100%), blaSHV (42.1%), tetA (60.5%), and sul1 (42.1%). 63.1% of recovered isolates were considered MDR, while 28.9% of recovered isolates were considered XDR. Some isolates harbor both virulence and MDR genes; the highest percentage carried 11, followed by isolates harboring 9 virulence and resistance genes. It could be concluded that the high prevalence of A. hydrophila in aquaculture species and their diverse antibiotic resistance and virulence genes suggest the high risk of Aeromonas infection and could have important implications for aquaculture and public health.

Muropeptides and muropeptide transporters impact on host immune response

In bacteria, the cell envelope is the key element surrounding and protecting the bacterial content from mechanical or osmotic damages. It allows the selective interchanges of solutes, ions, cellular debris, and drugs between the cellular compartments and the external environment, thanks to the presence of transmembrane proteins called transporters. The major component of the cell envelope is the peptidoglycan, consisting of long linear glycan strands cross-linked by short peptide stems. During cell growth or under stress conditions, peptidoglycan fragments, the muropeptides, are released by bacteria and recognized by the host Pattern Recognition Receptor, promoting the activation of their innate defense mechanisms. The review sums up the salient aspects of microbiota-host interaction with a focus on the NOD-dependent immune response to bacterial peptidoglycan and on the accountability of muropeptide transporters in the crosstalk with the host and in antibiotic resistance. Furthermore, it retraces the discoveries and applications of microorganisms-derived components such as vaccines or vaccine adjuvants.

Antibiotic Resistance, Virulence Gene Detection, and Biofilm Formation in Aeromonas spp. Isolated from Fish and Humans in Egypt

The genus Aeromonas is widely distributed in aquatic environments and is recognized as a potential human pathogen. Some Aeromonas species are able to cause a wide spectrum of diseases, mainly gastroenteritis, skin and soft-tissue infections, bacteremia, and sepsis. The aim of the current study was to determine the prevalence of Aeromonas spp. in raw fish markets and humans in Zagazig, Egypt; identify the factors that contribute to virulence; determine the isolates’ profile of antibiotic resistance; and to elucidate the ability of Aeromonas spp. to form biofilms. The examined samples included fish tissues and organs from tilapia (Oreochromis niloticus, n = 160) and mugil (Mugil cephalus, n = 105), and human skin swabs (n = 51) and fecal samples (n = 27). Based on biochemical and PCR assays, 11 isolates (3.2%) were confirmed as Aeromonas spp. and four isolates (1.2%) were confirmed as A. hydrophila. The virulence genes including haemolysin (hyl A) and aerolysin (aer) were detected using PCR in A. hydrophila in percentages of 25% and 50%, respectively. The antimicrobial resistance of Aeromonas spp. was assessed against 14 antibiotics comprising six classes. The resistance to cefixime (81.8%) and tobramycin (45.4%) was observed. The multiple antibiotic resistance (MAR) index ranged between 0.142–0.642 with 64.2% of the isolates having MAR values equal to 0.642. Biofilm formation capacity was assessed using a microtiter plate assay, and two isolates (18.1%) were classified as biofilm producers. This study establishes a baseline for monitoring and controlling the multidrug-resistant Aeromonas spp. and especially A. hydrophila in marine foods consumed in our country to protect humans and animals.

Bacterial virulence regulation through soluble peptidoglycan fragments sensing and response: knowledge gaps and therapeutic potential

Given the growing clinical-epidemiological threat posed by the phenomenon of antibiotic resistance, new therapeutic options are urgently needed, especially against top nosocomial pathogens such as those within the ESKAPE group. In this scenario, research is pushed to explore therapeutic alternatives and, among these, those oriented toward reducing bacterial pathogenic power could pose encouraging options. However, the first step in developing these antivirulence weapons is to find weak points in the bacterial biology to be attacked with the goal of dampening pathogenesis. In this regard, during the last decades some studies have directly/indirectly suggested that certain soluble peptidoglycan-derived fragments display virulence-regulatory capacities, likely through similar mechanisms to those followed to regulate the production of several β-lactamases: binding to specific transcriptional regulators and/or sensing/activation of two-component systems. These data suggest the existence of intra- and also intercellular peptidoglycan-derived signaling capable of impacting bacterial behavior, and hence likely exploitable from the therapeutic perspective. Using the well-known phenomenon of peptidoglycan metabolism-linked β-lactamase regulation as a starting point, we gather and integrate the studies connecting soluble peptidoglycan sensing with fitness/virulence regulation in Gram-negatives, dissecting the gaps in current knowledge that need filling to enable potential therapeutic strategy development, a topic which is also finally discussed.

Prevalence and genetic diversity of Aeromonas veronii isolated from aquaculture systems in the Poyang Lake area, China

The area around Poyang Lake is the main aquaculture area in Jiangxi Province, China, and an important base for the supply of freshwater aquatic products. Aquaculture in the Poyang Lake area is severely threatened by diseases caused by bacterial pathogens, and Aeromonas veronii has been the main pathogen in recent years. In this paper, ERIC-PCR genotyping, virulence gene and antimicrobial resistance gene detection, and drug susceptibility tests were carried out on 46 A. veronii isolates obtained from aquaculture systems in the Poyang Lake area from 2016 to 2020. The results showed that the A. veronii strains in the Poyang Lake area had high genetic diversity, and 46 strains produced 36 ERIC genotypes. There were no geographical and temporal differences in the cluster analysis results and no dominant clones. All 13 virulence genes tested were detected, and all isolates had harbored 2 or more virulence genes, with a maximum of 12 virulence genes detected. Among the 22 antimicrobial resistance genes selected, 15 were detected; 97.8% of the isolates contained 2 or more antimicrobial resistance genes, with a maximum of 9 antimicrobial resistance genes. Drug susceptibility tests showed that some strains were resistant to a variety of traditionally effective drugs for Aeromomas, such as enrofloxacin and florfenicol. This study provides a reference for exploring the impact of aquaculture in the Poyang Lake area on public health.

Bacterial Stress Responses as Potential Targets in Overcoming Antibiotic Resistance

Bacteria can be adapted to adverse and detrimental conditions that induce general and specific responses to DNA damage as well as acid, heat, cold, starvation, oxidative, envelope, and osmotic stresses. The stress-triggered regulatory systems are involved in bacterial survival processes, such as adaptation, physiological changes, virulence potential, and antibiotic resistance. Antibiotic susceptibility to several antibiotics is reduced due to the activation of stress responses in cellular physiology by the stimulation of resistance mechanisms, the promotion of a resistant lifestyle (biofilm or persistence), and/or the induction of resistance mutations. Hence, the activation of bacterial stress responses poses a serious threat to the efficacy and clinical success of antibiotic therapy. Bacterial stress responses can be potential targets for therapeutic alternatives to antibiotics. An understanding of the regulation of stress response in association with antibiotic resistance provides useful information for the discovery of novel antimicrobial adjuvants and the development of effective therapeutic strategies to control antibiotic resistance in bacteria. Therefore, this review discusses bacterial stress responses linked to antibiotic resistance in Gram-negative bacteria and also provides information on novel therapies targeting bacterial stress responses that have been identified as potential candidates for the effective control of Gram-negative antibiotic-resistant bacteria.

Aquatic environments: A Potential Source of Antimicrobial-Resistant Vibrio spp

Vibrio spp. are associated with water and seafood-related outbreaks worldwide. They are naturally present in aquatic environments such as seawater, brackish water and freshwater environments. These aquatic environments serve as the main reservoirs of antimicrobial-resistant genes and promote the transfer of antimicrobial-resistant bacterial species to aquatic animals and humans through the aquatic food chain. Vibrio spp. are known as etiological agents of cholera and non-cholera Vibrio infections in humans and animals. Antimicrobial-resistant Vibrio species have become a huge threat in regard to treating Vibrio infections in aquaculture and public health. Most of the Vibrio spp. possess resistance towards the commonly used antimicrobials, including β-lactams, aminoglycosides, tetracyclines, sulfonamides, quinolones and macrolides. The aim of this review is to summarize the antimicrobial resistance properties of Vibrio spp. isolated from aquatic environments to provide awareness about potential health risks related to Vibrio infections in aquaculture and public health.

Prevalence, antibiotic susceptibility, and presence of drug resistance genes in Aeromonas spp. isolated from freshwater fish in Kelantan and Terengganu states, Malaysia

Background and Aim:

The emergence of antibiotic-resistant bacterial pathogens has been increasingly reported, which has resulted in a decreasing ability to treat bacterial infections. Therefore, this study investigated the presence of Aeromonas spp., including its antibiotic resistance in various fish samples, Oreochromis spp., Clarias gariepinus, and Pangasius hypophthalmus, obtained from Kelantan and Terengganu, Malaysia.

Materials and Methods:

In this study, 221 fish samples, of which 108 (Oreochromis spp., n=38; C. gariepinus, n=35; and P. hypophthalmus, n=35) were from Kelantan and 113 (Oreochromis spp., n=38; C. gariepinus, n=35; and P. hypophthalmus, n=40) were from Terengganu, were caught using cast nets. Then, samples from their kidneys were cultured on a Rimler Shott agar to isolate Aeromonas spp. Polymerase chain reaction (PCR) was used to confirm this isolation using specific gene primers for species identification. Subsequently, the isolates were tested for their sensitivity to 14 antibiotics using the Kirby–Bauer method, after which the PCR was conducted again to detect resistance genes: sul1, strA-strB, aadA, bla_TEM, bla_SHV, tetA-tetE, and tetM.

Results:

From the results, 61 isolates were identified as being from the genus Aeromonas using PCR, of which 28 were Aeromonas jandaei, 19 were Aeromonas veronii, seven were Aeromonas hydrophila, and seven were Aeromonas sobria. Moreover, 8, 12, and 8 of A. jandaei; 4, 3, and 12 of A. veronii; 6, 0, and 1 of A. hydrophila; and 3, 3, and 1 of A. sobria were obtained from Oreochromis spp., C. gariepinus, and P. hypophthalmus, respectively. In addition, the isolates showed the highest level of resistance to ampicillin (100%), followed by streptomycin (59.0%), each kanamycin and nalidixic acid (41.0%), neomycin (36.1%), tetracycline (19.7%), sulfamethoxazole (14.8%), and oxytetracycline (13.1%). Resistance to gentamicin and ciprofloxacin both had the same percentage (9.8%), whereas isolates showed the lowest resistance to norfloxacin (8.2%) and doxycycline (1.6%). Notably, all Aeromonas isolates were susceptible to chloramphenicol and nitrofurantoin. Results also revealed that the multiple antibiotic resistances index of the isolates ranged from 0.07 to 0.64, suggesting that the farmed fish in these areas were introduced to the logged antibiotics indiscriminately and constantly during their cultivation stages. Results also revealed that the sul1 gene was detected in 19.7% of the Aeromonas isolates, whereas the tetracycline resistance genes, tetA and tetE, were detected in 27.9% and 4.9% of the isolates, respectively. However, β-lactam resistance genes, bla_TEM and bla_SHV, were found in 44.3% and 13.1% of Aeromonas isolates, respectively, whereas strA-strB and aadA genes were found in 3.3% and 13.1% of the isolates, respectively.

Conclusion:

This study, therefore, calls for continuous surveillance of antibiotic-resistant Aeromonas spp. in cultured freshwater fish to aid disease management and better understand their implications to public health.

Metallo-β-lactamases in the Age of Multidrug Resistance: From Structure and Mechanism to Evolution, Dissemination, and Inhibitor Design

Antimicrobial resistance is one of the major problems in current practical medicine. The spread of genes coding for resistance determinants among bacteria challenges the use of approved antibiotics, narrowing the options for treatment. Resistance to carbapenems, last resort antibiotics, is a major concern. Metallo-β-lactamases (MBLs) hydrolyze carbapenems, penicillins, and cephalosporins, becoming central to this problem. These enzymes diverge with respect to serine-β-lactamases by exhibiting a different fold, active site, and catalytic features. Elucidating their catalytic mechanism has been a big challenge in the field that has limited the development of useful inhibitors. This review covers exhaustively the details of the active-site chemistries, the diversity of MBL alleles, the catalytic mechanism against different substrates, and how this information has helped developing inhibitors. We also discuss here different aspects critical to understand the success of MBLs in conferring resistance: the molecular determinants of their dissemination, their cell physiology, from the biogenesis to the processing involved in the transit to the periplasm, and the uptake of the Zn(II) ions upon metal starvation conditions, such as those encountered during an infection. In this regard, the chemical, biochemical and microbiological aspects provide an integrative view of the current knowledge of MBLs.

Role of the PhoPQ two-component regulatory system in the β-lactam resistance of Stenotrophomonas maltophilia

BACKGROUND

Stenotrophomonas maltophilia, an opportunistic pathogen, is intrinsically resistant to most β-lactams except ceftazidime and ticarcillin/clavulanate, due to the inducibly expressed L1 and L2 β-lactamases. A two-component regulatory system (TCS) allows organisms to sense and respond to changes in different environmental conditions. The PhoPQ TCS of S. maltophilia plays regulatory roles in antibiotic susceptibility, physiology, stress adaption and virulence. Inactivation of S. maltophilia phoPQ increases β-lactam susceptibility.

OBJECTIVES

To elucidate the PhoPQ-regulating mechanism for β-lactam resistance.

METHODS

The candidate genes responsible for the ΔphoPQ-mediated β-lactam resistance compromise were identified by transcriptome analysis and verified by quantitative RT-PCR and complementation assay. Etest was used to assess β-lactam susceptibility. The phosphorylation level of the PhoP protein was determined by Phos-tag SDS-PAGE and western blotting. A β-lactam influx assay was used to investigate the influx efficiency of a β-lactam.

RESULTS

PhoPQ deletion down-regulated the expression of mltD1 and slt, attenuated the induced β-lactamase activity and then compromised the β-lactam resistance. Complementation of mutant phoPQ with mltD1 or slt genes partially reverted the induced β-lactamase activity and β-lactam resistance. The PhoPQ TCS was activated in logarithmically grown KJ cells and was further activated by low magnesium, but not by a β-lactam. However, low-magnesium-mediated PhoPQ activation hardly made an impact on β-lactam resistance enhancement. Furthermore, PhoPQ inactivation altered the outer membrane permeability and increased the influx of a β-lactam.

CONCLUSIONS

The PhoPQ TCS is activated to some extent in physiologically grown S. maltophilia. Inactivation of phoPQ attenuates the expression of mltD1 and slt, and increases β-lactam influx, both synergically contributing to β-lactam resistance compromise.

Virulence and antimicrobial resistance potential of Aeromonas spp. associated with shellfish

Aeromonas spp. are associated with seafood-related outbreaks worldwide. In seafood industry, shellfish play a major role in global seafood production. With this emerging trend of shellfish consumption, shellfish-related bacterial infections are being reported frequently. Aeromonas spp. are natural contaminants found in shellfish. Although 36 species have been identified, some species including Aeromonas hydrophila, Aeromonas caviae and Aeromonas veronii biotype sobria have dragged major attention as foodborne pathogenic bacteria. The ability to elaborate a variety of virulence factors of Aeromonas spp. contributes to the pathogenic activities. Also, emerging antimicrobial resistance in Aeromonas spp. has become a huge challenge in seafood industry. Furthermore, multidrug resistance increases the risk of consumer health. Studies have supplied pieces of evidence about the emerging health risk of Aeromonas spp. isolated from seafood. Therefore, the present review was intended to highlight the prevalence, virulence and antimicrobial resistance of Aeromonas spp. isolated from various types of shellfish.

Ornamental fish: a potential source of pathogenic and multidrug-resistant motile Aeromonas spp

Aeromonas spp. are ubiquitous bacteria that cause diseases in fish and other aquatic animals. They are the natural inhabitants of different aquatic environments, such as freshwater, brackishwater and marinewater. Extrinsic stressors, such as crowding, unhygienic handling, poor water quality, polluted feeding and inadequate nutrition, can predispose fish to Aeromonas infection. In ornamental fish, motile Aeromonas spp. are known as aetiological agents of motile aeromonad infections, which cause significant mortality in fish and economic loss in the ornamental fish industry. The existence of different virulence factors leads to the virulence potential of motile Aeromonas spp. There are several antimicrobials used to treat bacterial infections in ornamental fish. However, the extensive use of antimicrobials in the ornamental fish industry causes multidrug resistance. This article reviewed a multitude of virulence factors that are related to the ornamental fish-borne Aeromonas pathogenicity and the antimicrobial resistance determinants related to the multidrug resistance phenotypes of motile Aeromonas spp. in ornamental fish.

Regulation of AmpC-Driven β-Lactam Resistance in Pseudomonas aeruginosa: Different Pathways, Different Signaling

The hyperproduction of the chromosomal AmpC β-lactamase is the main mechanism driving β-lactam resistance in Pseudomonas aeruginosa, one of the leading opportunistic pathogens causing nosocomial acute and chronic infections in patients with underlying respiratory diseases. In the current scenario of the shortage of effective antipseudomonal drugs, understanding the molecular mechanisms mediating AmpC hyperproduction in order to develop new therapeutics against this fearsome pathogen is of great importance. It has been accepted for decades that certain cell wall-derived soluble fragments (muropeptides) modulate AmpC production by complexing with the transcriptional regulator AmpR and acquiring different conformations that activate/repress ampC expression. However, these peptidoglycan-derived signals have never been characterized in the highly prevalent P. aeruginosa stable AmpC hyperproducer mutants. Here, we demonstrate that the previously described fragments enabling the transient ampC hyperexpression during cefoxitin induction (1,6-anhydro-N-acetylmuramyl-pentapeptides) also underlie the dacB (penicillin binding protein 4 [PBP4]) mutation-driven stable hyperproduction but differ from the 1,6-anhydro-N-acetylmuramyl-tripeptides notably overaccumulated in the ampD knockout mutant. In addition, a simultaneous greater accumulation of both activators appears linked to higher levels of AmpC hyperproduction, although our results suggest a much stronger AmpC-activating potency for the 1,6-anhydro-N-acetylmuramyl-pentapeptide. Collectively, our results propose a model of AmpC control where the activator fragments, with qualitative and quantitative particularities depending on the pathways and levels of β-lactamase production, dominate over the repressor (UDP-N-acetylmuramyl-pentapeptide). This study represents a major step in understanding the foundations of AmpC-dependent β-lactam resistance in P. aeruginosa, potentially useful to open new therapeutic conceptions intended to interfere with the abovementioned cell wall-derived signaling.IMPORTANCE The extensive use of β-lactam antibiotics and the bacterial adaptive capacity have led to the apparently unstoppable increase of antimicrobial resistance, one of the current major global health challenges. In the leading nosocomial pathogen Pseudomonas aeruginosa, the mutation-driven AmpC β-lactamase hyperproduction stands out as the main resistance mechanism, but the molecular cues enabling this system have remained elusive until now. Here, we provide for the first time direct and quantitative information about the soluble cell wall-derived fragments accounting for the different levels and pathways of AmpC hyperproduction. Based on these results, we propose a hierarchical model of signals which ultimately govern ampC hyperexpression and resistance.

Phylogenetic characteristics, virulence properties and antibiogram profile of motile Aeromonas spp. isolated from ornamental guppy (Poecilia reticulata)

Aeromonas spp. are opportunistic pathogenic bacteria related to an assembly of infectious diseases in ornamental fish. In the present study, virulence properties and antibiotic susceptibility of 52 guppy-borne Aeromonas spp. were investigated. The isolates were identified as A. veronii (n = 34), A. dhakensis (n = 10), A. hydrophila (n = 3), A. caviae (n = 3) and A. enteropelogenes (n = 2) by gyrB gene sequencing. The gyrB sequence deviation within and among the species ranged from 0 to 2.6% and 2.7-9.2%. Each species formed a distinct group in the unrooted neighbor-joining phylogenetic tree. The phenotypic virulence factors such as β-hemolysis, slime, caseinase, DNase, gelatinase and lipase production were observed in 28 (53.9%), 33 (63.5%), 28 (53.9%), 42 (80.8%), 37 (71.2%) and 42 (80.8%) isolates, respectively. The virulence genes were detected by PCR assay in the following proportions- act (84.6%), hly (80.8%), aer (73.1%), lip (73.1%), gcaT (73.1%), ascV (53.8%), ahyB (53.8%) fla (51.9%), alt (48.1%), ast (36.5%) and ser (34.6%), respectively. The amoxicillin, ampicillin, imipenem, nalidixic acid, oxytetracycline and rifampicin were resistant to more than 70.0% of the isolates in antibiotic susceptibility test. Our study suggests that the ornamental guppy can be a potential reservoir of virulent and multi-drug resistant Aeromonas spp.

CreC Sensor Kinase Activation Enhances Growth of Escherichia coli in the Presence of Cephalosporins and Carbapenems

Mutants with enhanced growth in the presence of an antibiotic are more difficult to identify than mutants where the antibiotic's MIC increases, because they are not amenable to lethal selection in vitro We report that activatory mutations in the CreC signal sensor enhance growth of Escherichia coli in the presence of cefoxitin, cefotaxime, and meropenem, without increasing their MICs. Enhanced growth is dependent on overproduction of the inner membrane cre regulon protein CreD.

Roles of two-component regulatory systems in antibiotic resistance

Two-component regulatory systems (TCSs) are a major mechanism by which bacteria sense and respond to changes in their environment. TCSs typically consist of two proteins that bring about major regulation of the cell genome through coordinated action mediated by phosphorylation. Environmental conditions that activate TCSs are numerous and diverse and include exposure to antibiotics as well as conditions inside a host. The resulting regulatory action often involves activation of antibiotic defenses and changes to cell physiology that increase antibiotic resistance. Examples of resistance mechanisms enacted by TCSs contained in this review span those found in both Gram-negative and Gram-positive species and include cell surface modifications, changes in cell permeability, increased biofilm formation, and upregulation of antibiotic-degrading enzymes.

Peptidoglycan Muropeptides: Release, Perception, and Functions as Signaling Molecules

Peptidoglycan (PG) is an essential molecule for the survival of bacteria, and thus, its biosynthesis and remodeling have always been in the spotlight when it comes to the development of antibiotics. The peptidoglycan polymer provides a protective function in bacteria, but at the same time is continuously subjected to editing activities that in some cases lead to the release of peptidoglycan fragments (i.e., muropeptides) to the environment. Several soluble muropeptides have been reported to work as signaling molecules. In this review, we summarize the mechanisms involved in muropeptide release (PG breakdown and PG recycling) and describe the known PG-receptor proteins responsible for PG sensing. Furthermore, we overview the role of muropeptides as signaling molecules, focusing on the microbial responses and their functions in the host beyond their immunostimulatory activity.

Diversity and regulation of intrinsic β-lactamases from non-fermenting and other Gram-negative opportunistic pathogens

This review deeply addresses for the first time the diversity, regulation and mechanisms leading to mutational overexpression of intrinsic β-lactamases from non-fermenting and other non-Enterobacteriaceae Gram-negative opportunistic pathogens. After a general overview of the intrinsic β-lactamases described so far in these microorganisms, including circa. 60 species and 100 different enzymes, we review the wide array of regulatory pathways of these β-lactamases. They include diverse LysR-type regulators, which control the expression of β-lactamases from relevant nosocomial pathogens such as Pseudomonas aeruginosa or Stenothrophomonas maltophilia or two-component regulators, with special relevance in Aeromonas spp., along with other pathways. Likewise, the multiple mutational mechanisms leading to β-lactamase overexpression and β-lactam resistance development, including AmpD (N-acetyl-muramyl-L-alanine amidase), DacB (PBP4), MrcA (PPBP1A) and other PBPs, BlrAB (two-component regulator) or several lytic transglycosylases among others, are also described. Moreover, we address the growing evidence of a major interplay between β-lactamase regulation, peptidoglycan metabolism and virulence. Finally, we analyse recent works showing that blocking of peptidoglycan recycling (such as inhibition of NagZ or AmpG) might be useful to prevent and revert β-lactam resistance. Altogether, the provided information and the identified gaps should be valuable for guiding future strategies for combating multidrug-resistant Gram-negative pathogens.

Distribution of Antimicrobial Resistance Genes and Class 1 Integron Gene Cassette Arrays in Motile Aeromonas spp. Isolated from Goldfish (Carassius auratus)

Aeromonas spp. are opportunistic pathogens related to multiple infectious diseases in ornamental fishes. In the present study, the antimicrobial susceptibility, resistance genes, and integrons of 65 goldfish-borne Aeromonas spp. were evaluated. The isolates were identified as A. hydrophila (n = 30), A. veronii (n = 32), and A. punctata (n = 3) by gyrB sequencing. The antimicrobial susceptibility testing of the isolates designated that most of the isolates were resistant to amoxicillin (100.00%), nalidixic acid (100.00%), ampicillin (98.46%), tetracycline (92.31%), rifampicin (86.15%), and cephalothin (61.54%) and each of the isolates showed multiple antimicrobial resistance phenotype (resistant to ≥3 classes of antimicrobials). PCR amplification of antimicrobial resistance genes revealed that the plasmid-mediated quinolone resistance gene, qnrS, was the most prevalent (73.85%) among the isolates. The other antimicrobial resistance genes were detected in the following proportions: qnrB (26.15%), aac(6')-Ib-cr (4.60%), tetA (16.92%), tetE (21.54%), aac(6')-Ib (29.23%), and aphAI-IAB (7.69%). The IntI gene was found in 64.62% isolates, and four class 1 integron gene cassette profiles (incomplete dfrA1, catB3-aadA1, dfrA1-orfC, and qacE2-orfD) were identified. These data suggest that goldfish-borne Aeromonas spp. serve as a reservoir of antimicrobial resistance genes and class 1 integrons.

Impacts of Penicillin Binding Protein 2 Inactivation on β-Lactamase Expression and Muropeptide Profile in Stenotrophomonas maltophilia

Inducible expression of chromosomally encoded β-lactamase(s) is a key mechanism for β-lactam resistance in Enterobacter cloacae, Citrobacter freundii, Pseudomonas aeruginosa, and Stenotrophomonas maltophilia. The muropeptides produced during the peptidoglycan recycling pathway act as activator ligands for β-lactamase(s) induction. The muropeptides 1,6-anhydromuramyl pentapeptide and 1,6-anhydromuramyl tripeptide are the known activator ligands for ampC β-lactamase expression in E. cloacae. Here, we dissected the type of muropepetides for L1/L2 β-lactamase expression in an mrdA deletion mutant of S. maltophilia. Distinct from the findings with the ampC system, 1,6-anhydromuramyl tetrapeptide is the candidate for ΔmrdA-mediated β-lactamase expression in S. maltophilia. Our work extends the understanding of β-lactamase induction and provides valuable information for combating the occurrence of β-lactam resistance.

Penicillin binding proteins (PBPs) are involved in peptidoglycan synthesis, and their inactivation is linked to β-lactamase expression in ampR–β-lactamase module–harboring Gram-negative bacteria. There are seven annotated PBP genes, namely, mrcA, mrcB, pbpC, mrdA, ftsI, dacB, and dacC, in the Stenotrophomonas maltophilia genome, and these genes encode PBP1a, PBP1b, PBP1c, PBP2, PBP3, PBP4, and PBP6, respectively. In addition, S. maltophilia harbors two β-lactamase genes, L1 and L2, whose expression is induced via β-lactam challenge. The impact of PBP inactivation on L1/L2 expression was assessed in this study. Inactivation of mrdA resulted in increased L1/L2 expression in the absence of β-lactam challenge, and the underlying mechanism was further elucidated. The roles of ampNG, ampD_I (the homologue of Escherichia coli ampD), nagZ, ampR, and creBC in L1/L2 expression mediated by a ΔmrdA mutant strain were assessed via mutant construction and β-lactamase activity determinations. Furthermore, the strain ΔmrdA-mediated change in the muropeptide profile was assessed using liquid chromatography mass spectrometry (LC-MS). The mutant ΔmrdA-mediated L1/L2 expression relied on functional AmpNG, AmpR, and NagZ, was restricted by AmpD_I, and was less related to the CreBC two-component system. Inactivation of mrdA significantly increased the levels of total and periplasmic N-acetylglucosaminyl-1,6-anhydro-N-acetylmuramyl-l-alanyl-d-glutamyl-meso-diamnopimelic acid-d-alanine (GlcNAc-anhMurNAc tetrapeptide, or M4N), supporting that the critical activator ligands for mutant strain ΔmrdA-mediated L1/L2 expression are anhMurNAc tetrapeptides.

IMPORTANCE Inducible expression of chromosomally encoded β-lactamase(s) is a key mechanism for β-lactam resistance in Enterobacter cloacae, Citrobacter freundii, Pseudomonas aeruginosa, and Stenotrophomonas maltophilia. The muropeptides produced during the peptidoglycan recycling pathway act as activator ligands for β-lactamase(s) induction. The muropeptides 1,6-anhydromuramyl pentapeptide and 1,6-anhydromuramyl tripeptide are the known activator ligands for ampC β-lactamase expression in E. cloacae. Here, we dissected the type of muropepetides for L1/L2 β-lactamase expression in an mrdA deletion mutant of S. maltophilia. Distinct from the findings with the ampC system, 1,6-anhydromuramyl tetrapeptide is the candidate for ΔmrdA-mediated β-lactamase expression in S. maltophilia. Our work extends the understanding of β-lactamase induction and provides valuable information for combating the occurrence of β-lactam resistance.

Relationship of the CreBC two-component regulatory system and inner membrane protein CreD with swimming motility in Stenotrophomonas maltophilia

The CreBC two-component system (TCS) is a conserved regulatory system found in Escherichia coli, Aeromonas spp., Pseudomonas aeruginosa, and Stenotrophomonas maltophilia. In this study, we determined how CreBC TCS regulates secreted protease activities and swimming motility using creB, creC, and creBC in-frame deletion mutants (KJΔCreB, KJΔCreC, and KJΔBC) of S. maltophilia KJ. Compared to wild-type KJ, KJΔCreB had a comparable secreted protease activity; however, the secreted protease activities were obviously reduced in KJΔCreC and KJΔBC, suggesting that CreC works together with another unidentified response regulator (not CreB) to regulate secreted protease activity. Single gene inactivation of creB or creC resulted in mutants with an enhanced swimming motility, and this phenotype was exacerbated in a double mutant KJΔBC. To elucidate the underlying mechanism responsible for the ΔcreBC-mediated swimming enhancement, flagella morphology observation, RNA-seq based transcriptome assay, qRT-PCR, and membrane integrity and potential assessment were performed. Flagella morphological observation ruled out the possibility that swimming enhancement was due to altered flagella morphology. CreBC inactivation upregulated the expression of creD and flagella-associated genes encoding the basal body- and motor-associated proteins. Furthermore, KJΔBC had an increased membrane susceptibility to Triton X-100 and CreD upregulation in KJΔBC partially alleviated the compromise of membrane integrity. The impact of creBC TCS on bacterial membrane potential was assessed by carbonyl cyanide m-chlorophenyl hydrazine (CCCP50) concentration at which 50% of bacterial swimming is inhibited. CCCP50 of wild-type KJ increased when creBC was deleted, indicating an association between the higher membrane potential of KJΔBC cells and enhanced motility. Upregulation of the basal body- and motor-associated genes of flagella in KJΔBC cells may explain the increased membrane potential. Collectively, inactivation of creBC increased swimming motility through membrane potential increase and creD upregulation in S. maltophilia. The increased membrane potential may supply more energy for flagella propelling and CreD upregulation supports membrane stability, providing a strong membrane for flagellum function.

Pseudomonas aeruginosa: targeting cell-wall metabolism for new antibacterial discovery and development

Pseudomonas aeruginosa is a leading cause of hospital-acquired infections and is resistant to most antibiotics. With therapeutic options against P. aeruginosa dwindling, and the lack of new antibiotics in advanced developmental stages, strategies for preserving the effectiveness of current antibiotics are urgently required. β-Lactam antibiotics are important agents for treating P. aeruginosa infections, thus, adjuvants that potentiate the activity of these compounds are desirable for extending their lifespan while new antibiotics - or antibiotic classes - are discovered and developed. In this review, we discuss recent research that has identified exploitable targets of cell-wall metabolism for the design and development of compounds that hinder resistance and potentiate the activity of antipseudomonal β-lactams.

Expression and Functions of CreD, an Inner Membrane Protein in Stenotrophomonas maltophilia

CreBC is a highly conserved two-component regulatory system (TCS) in several gram-negative bacteria, including Escherichia coli, Aeromonas spp., Pseudomonas aeruginosa, and Stenotrophomonas maltophilia. CreD is a conserved gene that encodes a predicted inner-membrane protein and is located near the creBC loci. Activation of CreBC increases creD expression; therefore, creD expression is generally used as a measure of CreBC activation in E. coli, Aeromonas spp., and P. aeruginosa systems. In this article, we aim to elucidate the expression of creD and further to investigate its functions in S. maltophilia. In spite of a short intergenic region of 81 bp between creBC and creD, creD is expressed separately from the adjacent creBC operon and from a promoter immediately upstream of creD (P_creD) in S. maltophilia. We found that the promoter activity of P_creD is negatively regulated by the creBC TCS, positively regulated by the bacterial culture density, and not affected by β-lactams. Furthermore, creD expression is not significantly altered in the presence of the phosphor-mimic variant of CreB, CreB(D55E), which mimics activated CreB. The functions of CreD of S. maltophilia were assessed by comparison among the following: wild-type KJ; the creD isogenic mutant, KJΔCreD; and the complementary strain, KJΔCreD(pCreD). The mutant lacking creD had cell division defects and aberrations in cell envelope integrity, which then triggered the σ^E-mediated envelope stress response. Thus, the results indicated that CreD plays a critical role in the maintenance of envelope integrity.

Interplay among membrane-bound lytic transglycosylase D1, the CreBC two-component regulatory system, the AmpNG-AmpDI-NagZ-AmpR regulatory circuit, and L1/L2 β-lactamase expression in Stenotrophomonas maltophilia

Lytic transglycosylases (LTs) are an important class of enzymes involved in peptidoglycan (PG) cleavage, with the concomitant formation of an intramolecular 1,6-anhydromuramoyl reaction product. There are six annotated LT genes in the Stenotrophomonas maltophilia genome, including genes for five membrane-bound LTs (mltA, mltB1, mltB2, mltD1, and mltD2) and a gene for soluble LT (slt). Six LTs of S. maltophilia KJ were systematically mutated, yielding the ΔmltA, ΔmltB1, ΔmltB2, ΔmltD1, ΔmltD2, and Δslt mutants. Inactivation of mltD1 conferred a phenotype of elevated uninduced β-lactamase activity. The underlying mechanism responsible for this phenotype was elucidated by the construction of several mutants and determination of β-lactamase activity. The expression of the genes assayed was assessed by quantitative reverse transcriptase PCR and a promoter transcription fusion assay. The results demonstrate that ΔmltD1 mutant-mediated L1/L2 β-lactamase expression involved the creBC two-component regulatory system (TCS) and the ampNG-ampDI-nagZ-ampR regulatory circuit. The inactivation of mltD1 resulted in mltB1 and mltD2 upexpression in a creBC- and ampNG-dependent manner. The overexpressed MltB1 and MltD2 activity contributed to the expression of the L1/L2 β-lactamase genes via the ampNG-ampDI-nagZ-ampR regulatory circuit. These findings reveal, for the first time, a linkage between LTs, the CreBC TCS, the ampNG-ampDI-nagZ-ampR regulatory circuit, and L1/L2 β-lactamase expression in S. maltophilia.

Role of Pseudomonas aeruginosa low-molecular-mass penicillin-binding proteins in AmpC expression, β-lactam resistance, and peptidoglycan structure

This study aimed to characterize the role of Pseudomonas aeruginosa low-molecular-mass penicillin-binding proteins (LMM PBPs), namely, PBP4 (DacB), PBP5 (DacC), and PBP7 (PbpG), in peptidoglycan composition, β-lactam resistance, and ampC regulation. For this purpose, we constructed all single and multiple mutants of dacB, dacC, pbpG, and ampC from the wild-type P. aeruginosa PAO1 strain. Peptidoglycan composition was determined by high-performance liquid chromatography (HPLC), ampC expression by reverse transcription-PCR (RT-PCR), PBP patterns by a Bocillin FL-binding test, and antimicrobial susceptibility by MIC testing for a panel of β-lactams. Microscopy and growth rate analyses revealed no apparent major morphological changes for any of the mutants compared to the wild-type PAO1 strain. Of the single mutants, only dacC mutation led to significantly increased pentapeptide levels, showing that PBP5 is the major dd-carboxypeptidase in P. aeruginosa. Moreover, our results indicate that PBP4 and PBP7 play a significant role as dd-carboxypeptidase only if PBP5 is absent, and their dd-endopeptidase activity is also inferred. As expected, the inactivation of PBP4 led to a significant increase in ampC expression (around 50-fold), but, remarkably, the sequential inactivation of the three LMM PBPs produced a much greater increase (1,000-fold), which correlated with peptidoglycan pentapeptide levels. Finally, the β-lactam susceptibility profiles of the LMM PBP mutants correlated well with the ampC expression data. However, the inactivation of ampC in these mutants also evidenced a role of LMM PBPs, especially PBP5, in intrinsic β-lactam resistance. In summary, in addition to assessing the effect of P. aeruginosa LMM PBPs on peptidoglycan structure for the first time, we obtained results that represent a step forward in understanding the impact of these PBPs on β-lactam resistance, apparently driven by the interplay between their roles in AmpC induction, β-lactam trapping, and dd-carboxypeptidase/β-lactamase activity.

PBP1a/LpoA but not PBP1b/LpoB are involved in regulation of the major β-lactamase gene blaA in Shewanella oneidensis

β-Lactamase production is one of the most important strategies for Gram-negative bacteria to combat β-lactam antibiotics. Studies of the regulation of β-lactamase expression have largely been focused on the class C β-lactamase AmpC, whose induction by β-lactams requires LysR-type regulator AmpR and permease AmpG-dependent peptidoglycan recycling intermediates. In Shewanella, which is ubiquitous in aquatic environments and is a reservoir for antibiotic resistance, production of the class D β-lactamase BlaA confers bacteria with natural resistance to many β-lactams. Expression of the blaA gene in the genus representative Shewanella oneidensis is distinct from the AmpC paradigm because of the lack of an AmpR homologue and the presence of an additional AmpG-independent regulatory pathway. In this study, using transposon mutagenesis, we identify proteins that are involved in blaA regulation. Inactivation of mrcA and lpoA, which encode penicillin binding protein 1a (PBP1a) and its lipoprotein cofactor, LpoA, respectively, drastically enhances blaA expression in the absence of β-lactams. Although PBP1b and its cognate, LpoB, also exist in S. oneidensis, their roles in blaA induction are dispensable. We further show that the mrcA-mediated blaA expression is independent of AmpG.

The Pseudomonas aeruginosa CreBC two-component system plays a major role in the response to β-lactams, fitness, biofilm growth, and global regulation

Pseudomonas aeruginosa is a ubiquitous versatile environmental microorganism with a remarkable ability to grow under diverse environmental conditions. Moreover, P. aeruginosa is responsible for life-threatening infections in immunocompromised and cystic fibrosis patients, as the extraordinary capacity of this pathogen to develop antimicrobial resistance dramatically limits our therapeutic arsenal. Its large genome carries an outstanding number of genes belonging to regulatory systems, including multiple two-component sensor-regulator systems that modulate the response to the different environmental stimuli. Here, we show that one of two systems, designated CreBC (carbon source responsive) and BlrAB (β-lactam resistance), might be of particular relevance. We first identified the stimuli triggering the activation of the CreBC system, which specifically responds to penicillin-binding protein 4 (PBP4) inhibition by certain β-lactam antibiotics. Second, through an analysis of a large comprehensive collection of mutants, we demonstrate an intricate interconnection between the CreBC system, the peptidoglycan recycling pathway, and the expression of the concerning chromosomal β-lactamase AmpC. Third, we show that the CreBC system, and particularly its effector inner membrane protein CreD, plays a major role in bacterial fitness and biofilm development, especially in the presence of subinhibitory concentrations of β-lactams. Finally, global transcriptomics reveals broad regulatory functions of CreBC in basic physiological aspects, particularly anaerobic respiration, in both the presence and absence of antibiotics. Therefore, the CreBC system is envisaged as a potentially interesting target for improving the efficacy of β-lactams against P. aeruginosa infections.

The sentinel role of peptidoglycan recycling in the β-lactam resistance of the Gram-negative Enterobacteriaceae and Pseudomonas aeruginosa

The peptidoglycan is the structural polymer of the bacterial cell envelope. In contrast to an expectation of a structural stasis for this polymer, during the growth of the Gram-negative bacterium this polymer is in a constant state of remodeling and extension. Our current understanding of this peptidoglycan "turnover" intertwines with the deeply related phenomena of the liberation of small peptidoglycan segments (muropeptides) during turnover, the presence of dedicated recycling pathways for reuse of these muropeptides, β-lactam inactivation of specific penicillin-binding proteins as a mechanism for the perturbation of the muropeptide pool, and this perturbation as a controlling mechanism for signal transduction leading to the expression of β-lactamase(s) as a key resistance mechanism against the β-lactam antibiotics. The nexus for many of these events is the control of the AmpR transcription factor by the composition of the muropeptide pool generated during peptidoglycan recycling. In this review we connect the seminal observations of the past decades to new observations that resolve some, but certainly not all, of the key structures and mechanisms that connect to AmpR.

Resistance to antibiotics targeted to the bacterial cell wall

Peptidoglycan is the main component of the bacterial cell wall. It is a complex, three-dimensional mesh that surrounds the entire cell and is composed of strands of alternating glycan units crosslinked by short peptides. Its biosynthetic machinery has been, for the past five decades, a preferred target for the discovery of antibacterials. Synthesis of the peptidoglycan occurs sequentially within three cellular compartments (cytoplasm, membrane, and periplasm), and inhibitors of proteins that catalyze each stage have been identified, although not all are applicable for clinical use. A number of these antimicrobials, however, have been rendered inactive by resistance mechanisms. The employment of structural biology techniques has been instrumental in the understanding of such processes, as well as the development of strategies to overcome them. This review provides an overview of resistance mechanisms developed toward antibiotics that target bacterial cell wall precursors and its biosynthetic machinery. Strategies toward the development of novel inhibitors that could overcome resistance are also discussed.

AQU-1, a chromosomal class C β-lactamase, among clinical Aeromonas dhakensis isolates: distribution and clinical significance

Aeromonas dhakensis, a recently described Aeromonas sp. formerly called Aeromonas aquariorum, is associated with human infections. In this study, a chromosomal gene, blaAQU-1, was identified in A. dhakensis AAK1 that constitutes a 1143-bp open reading frame and is 87% identical to the gene encoding CepH in Aeromonas hydrophila. An Escherichia coli TOP10 cell transformant harbouring blaAQU-1 was resistant to cefotaxime but not to cefepime. mRNA expression of blaAQU-1 in the cefotaxime-resistant mutant strain AAK1m was 70-fold higher than in the wild strain AAK1. In all 16 A. dhakensis isolates (the major species of 51 consecutive Aeromonas blood isolates collected from June 1999 to June 2001) as well as in A. aquariorum MDC47(T) and A. hydrophila subsp. dhakensis LMG 19562(T), but not in the reference strains or clinical isolates of other A. hydrophila subspecies, Aeromonas caviae, Aeromonas veronii or Aeromonas enteropelogenes, blaAQU-1-related genes were detected by PCR. Overall, 13 (81%) of the 16 A. dhakensis blood isolates exhibited either cefotaxime resistance or the in vitro emergence of derepressed cefotaxime-resistant mutants. In vivo selection of an A. dhakensis resistant mutant was noted in a burn patient undergoing cefotaxime monotherapy. These observations suggest that AQU-1 is a chromosomal cephalosporinase in A. dhakensis. Cefotaxime monotherapy for severe A. dhakensis infections should be used cautiously.

Involvement of mutation in ampD I, mrcA, and at least one additional gene in β-lactamase hyperproduction in Stenotrophomonas maltophilia

It has been reported that targeted disruption of ampD I or mrcA causes β-lactamase hyperproduction in Stenotrophomonas maltophilia. We show here that β-lactamase-hyperproducing laboratory selected mutants and clinical isolates can have wild-type ampD I and mrcA genes, implicating mutation of at least one additional gene in this phenotype. The involvement of mutations at multiple loci in the activation of β-lactamase production in S. maltophilia reveals that there are significant deviations from the enterobacterial paradigm of AmpR-mediated control of β-lactamase induction. We do show, however, that S. maltophilia ampD I can complement a mutation in Escherichia coli ampD. This suggests that an anhydromuropeptide degradation product of peptidoglycan is used to activate AmpR in S. maltophilia, as is also the case in enteric bacteria.

Eliminating a set of four penicillin binding proteins triggers the Rcs phosphorelay and Cpx stress responses in Escherichia coli

Penicillin binding proteins (PBPs) are responsible for synthesizing and modifying the bacterial cell wall, and in Escherichia coli the loss of several nonessential low-molecular-weight PBPs gives rise to abnormalities in cell shape and division. To determine whether these proteins help connect the flagellar basal body to the peptidoglycan wall, we surveyed a set of PBP mutants and found that motility in an agar migration assay was compromised by the simultaneous absence of four enzymes: PBP4, PBP5, PBP7, and AmpH. A wild-type copy of any one of these restored migration, and complementation depended on the integrity of the PBP active-site serine. However, the migration defect was caused by the absence of flagella instead of improper flagellar assembly. Migration was restored if the flhDC genes were overexpressed or if the rcsB or cpxR genes were deleted. Thus, migration was inhibited because the Rcs and Cpx stress response systems were induced in the absence of these four specific PBPs. Furthermore, in this situation Rcs induction depended on the presence of CpxR. The results imply that small changes in peptidoglycan structure are sufficient to activate these stress responses, suggesting that a specific cell wall fragment may be the signal being sensed. The fact that four PBPs must be inactivated may explain why large perturbations to the envelope are required to induce stress responses.

Changes to its peptidoglycan-remodeling enzyme repertoire modulate β-lactam resistance in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a leading cause of hospital-acquired infections and is resistant to many antibiotics. Among its primary mechanisms of resistance is expression of a chromosomally encoded AmpC β-lactamase that inactivates β-lactams. The mechanisms leading to AmpC expression in P. aeruginosa remain incompletely understood but are intricately linked to cell wall metabolism. To better understand the roles of peptidoglycan-active enzymes in AmpC expression-and consequent β-lactam resistance-a phenotypic screen of P. aeruginosa mutants lacking such enzymes was performed. Mutants lacking one of four lytic transglycosylases (LTs) or the nonessential penicillin-binding protein PBP4 (dacB) had altered β-lactam resistance. mltF and slt mutants with reduced β-lactam resistance were designated WIMPs (wall-impaired mutant phenotypes), while highly resistant dacB, sltB1, and mltB mutants were designated HARMs (high-level AmpC resistant mutants). Double mutants lacking dacB and sltB1 had extreme piperacillin resistance (>256 μg/ml) compared to either of the single knockouts (64 μg/ml for a dacB mutant and 12 μg/ml for an sltB1 mutant). Inactivation of ampC reverted these mutants to wild-type susceptibility, confirming that AmpC expression underlies resistance. dacB mutants had constitutively elevated AmpC expression, but the LT mutants had wild-type levels of AmpC in the absence of antibiotic exposure. These data suggest that there are at least two different pathways leading to AmpC expression in P. aeruginosa and that their simultaneous activation leads to extreme β-lactam resistance.

Complexity of β-lactamases among clinical Aeromonas isolates and its clinical implications

Aeromonas species, aquatic Gram-negative bacilli, distributed globally and ubiquitously in the natural environment, may be implicated in a variety of human diseases. They can produce various β-lactamases which confer resistance to a broad spectrum of β-lactams, and therefore in vitro susceptibility testing must be used to guide antimicrobial therapy. However, conventional in vitro susceptibility tests may sometimes fail to detect these β-lactamases, and hence raise a therapeutic challenge. In this review article, two chromosomally mediated β-lactamases (i.e., AmpC β-lactamases and metallo-β-lactamases) and acquired extended-spectrum β-lactamases in aeromonads are reviewed, and the clinical implications of the complexity of β-lactamases are discussed.

Messenger functions of the bacterial cell wall-derived muropeptides

Bacterial muropeptides are soluble peptidoglycan structures central to recycling of the bacterial cell wall and messengers in diverse cell signaling events. Bacteria sense muropeptides as signals that antibiotics targeting cell-wall biosynthesis are present, and eukaryotes detect muropeptides during the innate immune response to bacterial infection. This review summarizes the roles of bacterial muropeptides as messengers, with a special emphasis on bacterial muropeptide structures and the relationship of structure to the biochemical events that the muropeptides elicit. Muropeptide sensing and recycling in both Gram-positive and Gram-negative bacteria are discussed, followed by muropeptide sensing by eukaryotes as a crucial event in the innate immune response of insects (via peptidoglycan-recognition proteins) and mammals (through Nod-like receptors) to bacterial invasion.

Expression of OXA-type and SFO-1 β-lactamases induces changes in peptidoglycan composition and affects bacterial fitness

β-Lactamases and penicillin-binding proteins (PBPs) have evolved from a common ancestor. β-Lactamases are enzymes that degrade β-lactam antibiotics, whereas PBPs are involved in the synthesis and processing of peptidoglycan, which forms an elastic network in the bacterial cell wall. This study analyzed the interaction between β-lactamases and peptidoglycan and the impact on fitness and biofilm production. A representative set of all classes of β-lactamases was cloned in the expression vector pBGS18 under the control of the CTX-M promoter and expressed in Escherichia coli MG1655. The peptidoglycan composition of all clones was evaluated, and quantitative changes were found in E. coli strains expressing OXA-24, OXA-10-like, and SFO-1 (with its upstream regulator AmpR) β-lactamases; the level of cross-linked muropeptides decreased, and their average length increased. These changes were associated with a statistically significant fitness cost, which was demonstrated in both in vitro and in vivo experiments. The observed changes in peptidoglycan may be explained by the presence of residual DD-endopeptidase activity in these β-lactamases, which may result in hydrolysis of the peptide cross bridge. The biological cost associated with these changes provides important data regarding the interaction between β-lactamases and the metabolism of peptidoglycan and may provide an explanation for the epidemiology of these β-lactamases in Enterobacteriaceae.

Analysis of genes encoding penicillin-binding proteins in clinical isolates of Acinetobacter baumannii

There is limited information on the role of penicillin-binding proteins (PBPs) in the resistance of Acinetobacter baumannii to β-lactams. This study presents an analysis of the allelic variations of PBP genes in A. baumannii isolates. Twenty-six A. baumannii clinical isolates (susceptible or resistant to carbapenems) from three teaching hospitals in Spain were included. The antimicrobial susceptibility profile, clonal pattern, and genomic species identification were also evaluated. Based on the six complete genomes of A. baumannii, the PBP genes were identified, and primers were designed for each gene. The nucleotide sequences of the genes identified that encode PBPs and the corresponding amino acid sequences were compared with those of ATCC 17978. Seven PBP genes and one monofunctional transglycosylase (MGT) gene were identified in the six genomes, encoding (i) four high-molecular-mass proteins (two of class A, PBP1a [ponA] and PBP1b [mrcB], and two of class B, PBP2 [pbpA or mrdA] and PBP3 [ftsI]), (ii) three low-molecular-mass proteins (two of type 5, PBP5/6 [dacC] and PBP6b [dacD], and one of type 7 (PBP7/8 [pbpG]), and (iii) a monofunctional enzyme (MtgA [mtgA]). Hot spot mutation regions were observed, although most of the allelic changes found translated into silent mutations. The amino acid consensus sequences corresponding to the PBP genes in the genomes and the clinical isolates were highly conserved. The changes found in amino acid sequences were associated with concrete clonal patterns but were not directly related to susceptibility or resistance to β-lactams. An insertion sequence disrupting the gene encoding PBP6b was identified in an endemic carbapenem-resistant clone in one of the participant hospitals.

Inactivation of mrcA gene derepresses the basal-level expression of L1 and L2 β-lactamases in Stenotrophomonas maltophilia

OBJECTIVES

To characterize the relationship between inactivation of the mrcA gene and β-lactamase expression and β-lactams resistance in Stenotrophomonas maltophilia KJ and to investigate the involvement of ampR, ampN-ampG, ampD(I) and creBC in this.

METHODS

The mrcA deletion mutant KJΔmrcA was constructed to investigate the role of this putative penicillin-binding protein 1a (PBP1a) in β-lactamase expression and β-lactam resistance. The ΔampR, ΔampNG, ΔampDI and ΔcreBC alleles were introduced into KJΔmrcA, and KJΔDIΔBC and KJΔDIΔmrcAΔBC were also constructed for comparison. All the mutants and their corresponding parent strains were assayed for β-lactamase activities and MICs of β-lactams.

RESULTS

Inactivation of mrcA caused basal L1/L2 β-lactamase production to increase by ∼100-fold, but made little difference to cefuroxime-induced β-lactamase activity and the MICs of β-lactams. The ΔmrcA-derived basal β-lactamase hyperproduction was ampR and ampN-ampG dependent. Simultaneous inactivation of ampD(I) and mrcA did not augment β-lactamase production over and above that seen in an ampD(I) mutant alone. Furthermore, we could find no evidence for a role of the creBC two-component regulatory system in β-lactamase hyperproduction in a ΔampD(I) or ΔmrcA background.

CONCLUSIONS

Inactivation of mrcA, predicted to encode PBP1a, causes basal L1/L2 β-lactamase hyperproduction in S. maltophilia.