Fluoroquinolone resistance linked to GyrA, GyrB, and ParC mutations in Salmonella enterica typhimurium isolates in humans

Mutational Diversity in the Quinolone Resistance-Determining Regions of Type-II Topoisomerases of Salmonella Serovars

Quinolone resistance in bacterial pathogens has primarily been associated with mutations in the quinolone resistance-determining regions (QRDRs) of bacterial type-II topoisomerases, which are DNA gyrase and topoisomerase IV. Depending on the position and type of the mutation (s) in the QRDRs, bacteria either become partially or completely resistant to quinolone. QRDR mutations have been identified and characterized in Salmonella enterica isolates from around the globe, particularly during the last decade, and efforts have been made to understand the propensity of different serovars to carry such mutations. Because there is currently no thorough analysis of the available literature on QRDR mutations in different Salmonella serovars, this review aims to provide a comprehensive picture of the mutational diversity in QRDRs of Salmonella serovars, summarizing the literature related to both typhoidal and non-typhoidal Salmonella serovars with a special emphasis on recent findings. This review will also discuss plasmid-mediated quinolone-resistance determinants with respect to their additive or synergistic contributions with QRDR mutations in imparting elevated quinolone resistance. Finally, the review will assess the contribution of membrane transporter-mediated quinolone efflux to quinolone resistance in strains carrying QRDR mutations. This information should be helpful to guide the routine surveillance of foodborne Salmonella serovars, especially with respect to their spread across countries, as well as to improve laboratory diagnosis of quinolone-resistant Salmonella strains.

Presence of plasmid-mediated quinolone resistance (PMQR) genes in non-typhoidal Salmonella strains with reduced susceptibility to fluoroquinolones isolated from human salmonellosis in Gyeonggi-do, South Korea from 2016 to 2019

Non-typhoidal salmonellosis remains a pressing public health problem worldwide. Quinolones, particularly fluoroquinolones, are widely used to treat various infections, including non-typhoidal salmonellosis, which can be a serious illness. The emergence of fluoroquinolone-resistant Salmonella has resulted in treatment failure and high mortality rates. In this study, we estimated the presence of plasmid-mediated quinolone resistance (PMQR) genes in Salmonella enterica isolated from human salmonellosis patients in South Korea from 2016 to 2019. We evaluated the association of these genes with fluoroquinolone susceptibility. Antimicrobial susceptibility tests for Salmonella isolates were performed using the Vitek II system, and the minimum inhibitory concentrations (MIC) of ciprofloxacin and levofloxacin were determined using the E-test method. Plasmid-mediated quinolone resistance (PMQR) genes were detected by PCR amplification and quinolone resistance-determining regions (QRDRs) of the gyrA and parC genes were analyzed following Sanger sequencing of the PCR products. Thirty-four Salmonella strains with reduced susceptibility to fluoroquinolones (ciprofloxacin MIC ≥ 0.125 µg/mL and levofloxacin MIC ≥ 0.25 µg/mL) were selected from 208 human clinical Salmonella isolates. Among them, 22 Salmonella strains harbored one PMQR gene (qnrA, qnrB, or qnrS), and three Salmonella strains carried two PMQR genes (qnrS and aac(6′)-Ib-cr or qnrA and qnrB). qnrS was the most common PMQR gene. Serotyping revealed that Salmonella 4,[5]12:i:- (32.4%, 11/34) and Salmonella Typhimurium (29.4%, 10/34) were the two most predominant serovars, and Multi-locus sequence typing (MLST) showed that ST19 and ST34 were the most frequent sequence types. In conclusion, qnr gene-positive Salmonella 4,[5],12:i:- and Salmonella Typhimurium were the main serovars responsible for reduced susceptibility to fluoroquinolones. Therefore, our findings suggest that PMQR-positive Salmonella strains, which can be isolated from various samples including human, food, and the environment, should be carefully monitored.

Antimicrobial Susceptibility and Genomic Analysis of Aliarcobacter cibarius and Aliarcobacter thereius, Two Rarely Detected Aliarcobacter Species

Aliarcobacter cibarius and Aliarcobacter thereius are two rarely detected Aliarcobacter species. In the study, we analyzed the antimicrobial susceptibility and provide detailed insights into the genotype and phylogeny of both species using whole-genome sequencing. Thermophilic Campylobacter species are the most common bacterial foodborne pathogens causing gastroenteritis in humans worldwide. The genus Aliarcobacter is part of the Campylobacteraceae family and includes the species Aliarcobacter butzleri, Aliarcobacter cryaerophilus, Aliarcobacter skirrowii, and the rarely described Aliarcobacter cibarius, Aliarcobacter faecis, Aliarcobacter lanthieri, Aliarcobacter thereius, and Acrobarter trophiarum. Aliarcobacter are emergent enteropathogens and potential zoonotic agents. Here, we generated, analyzed, and characterized whole-genome sequences of Aliarcobacter cibarius and Aliarcobacter thereius. They were isolated from water poultry farms in Germany, cultured and identified by MALDI-TOF MS. With PCR the identity was verified. Antibiotic susceptibility testing was carried out with erythromycin, ciprofloxacin, doxycycline, tetracycline, gentamicin, streptomycin, ampicillin, and cefotaxime using the gradient strip method (E-test). Whole-genome sequences were generated including those of reference strains. Complete genomes for six selected strains are reported. These provide detailed insights into the genotype. With these, we predicted in silico known AMR genes, virulence-associated genes, and plasmid replicons. Phenotypic analysis of resistance showed differences between the presence of resistance genes and the prediction of phenotypic resistance profiles. In Aliarcobacter butzleri, the nucleotide sequence of the gyrA gene (DQ464331) can show a signature mutation resulting in an amino acid change T85>I. Acrobarter cibarius and Acrobarter thereius showed the same gene as assessed by similarity annotation of the mutations 254C>G. Most of the isolates were found to be sensitive to ciprofloxacin. The ciprofloxacin-resistant Aliarcobacter thereius isolate was associated with the amino acid change T85>I. But this was not predicted with antibiotic resistance databases, before. Ultimately, a phylogenetic analysis was done to facilitate in future outbreak analysis.

Occurrence, quantification, pulse types, and antimicrobial susceptibility of Salmonella sp. isolated from chicken meat in the state of Paraná, Brazil

The aim of this work was to verify the occurrence, quantification, pulse types, and antimicrobial susceptibility profiles of Salmonella sp. isolated from chicken meat produced and marketed in the state of Paraná, considered to be the state with the highest production of poultry meat in Brazil. Ninety-five of 300 (31.5%) frozen cuts of chicken were found to contain Salmonella sp., and 98 different isolates of Salmonella sp. were cultured from the positive samples. Quantification showed low Salmonella sp. loading, ranging from 0.12 to 6.4 MPN/g. The antimicrobial resistance test was performed against 16 agents from 6 different classes. All isolates were sensitive to meropenem, imipenem, chloramphenicol, and amikacin. The highest resistance rates were observed for nalidixic acid (95%), tetracycline (94%), doxycycline (94%), ampicillin (87%), amoxicillin with clavulanic acid (84%), ceftriaxone (79%), and ciprofloxacin (76%). A total of 84 (85.7%) of the isolates were identified with a multidrug resistant profile, 13 of which were found to have encoding genes extended-spectrum beta-lactamase (ESBL), especially blaCTX-M-2 e blaTEM-1. The major serovars identified were S. Typhimurium (43%) and S. Heidelberg (39%). The third most isolated serovar was S. Ndolo (6%), without previous reports of its presence in poultry meat in Brazil. Molecular characterization of S. Typhimurium and S. Heidelberg isolates by pulsed field gel electrophoresis (PFGE) showed a clonal relationship between all isolates of the same serovar (genetic similarity greater than 80%). Isolates of S. Typhimurium and S. Heidelberg with 100% similarity were found in up to five different geographic regions of the state, showing the potential for the spread of this pathogen in the Paraná poultry chain. Epidemiological surveys like this are important to understand the dynamics of dissemination and to monitor the prevalence of pathogens in the final products of poultry chains. In addition, to know the resistance profile of strains of Salmonella sp. present in food that contributes to the adoption of faster and more effective therapeutic measures, when necessary.

WQ-3810 exerts high inhibitory effect on quinolone-resistant DNA gyrase of Salmonella Typhimurium

The inhibitory effect of WQ-3810 on DNA gyrase was assayed to evaluate the potential of WQ-3810 as a candidate drug for the treatment of quinolone resistant Salmonella Typhymurium infection. The inhibitory effect of WQ-3810, ciprofloxacin and nalidixic acid was compared by accessing the drug concentration that halves the enzyme activity (IC₅₀) of purified S. Typhimurium wildtype and mutant DNA gyrase with amino acid substitution at position 83 or/and 87 in subunit A (GyrA) causing quinolone resistance. As a result, WQ-3810 reduced the enzyme activity of both wildtype and mutant DNA gyrase at a lower concentration than ciprofloxacin and nalidixic acid. Remarkably, WQ-3810 showed a higher inhibitory effect on DNA gyrase with amino acid substitutions at position 87 than with that at position 83 in GyrA. This study revealed that WQ-3810 could be an effective therapeutic agent, especially against quinolone resistant Salmonella enterica having amino acid substitution at position 87.

Norfloxacin salts of carboxylic acids curtail planktonic and biofilm mode of growth in ESKAPE pathogens

AIMS

To enhance the antimicrobial and antibiofilm activity of norfloxacin against the planktonic and biofilm mode of growth in ESKAPE pathogens using chemically modified norfloxacin salts.

METHODS AND RESULTS

Antimicrobial testing, synergy testing and time-kill curve analysis were performed to evaluate antibacterial effect of norfloxacin carboxylic acid salts against ESKAPE pathogens. In vivo efficacy to reduce bacterial bioburden was evaluated in zebrafish infection model. Crystal violet assay and live-dead staining were performed to discern antibiofilm effect. Membrane permeability, integrity and molecular docking studies were carried out to ascertain the mechanism of action. The carboxylic acid salts, relative to parent molecule norfloxacin, displayed two- to fourfold reduction in minimum inhibitory concentration against Staphylococcus aureus and Pseudomonas aeruginosa, in addition to displaying potent bacteriostatic effect against certain members of ESKAPE pathogens. In vivo treatments revealed that norfloxacin tartrate (SRIN2) reduced MRSA bioburden by greater than 1 log fold relative to parent molecule in the muscle tissue. In silico docking with gyrA of S. aureus showed increased affinity of SRIN2 towards DNA gyrase. The enhanced antibacterial effect of norfloxacin salts could be partially accounted by altered membrane permeability in S. aureus and perturbed membrane integrity in P. aeruginosa. Antibiofilm studies revealed that SRIN2 (norfloxacin tartrate) and SRIN3 (norfloxacin benzoate) exerted potent antibiofilm effect particularly against Gram-negative ESKAPE pathogens. The impaired colonization of both S. aureus and P. aeruginosa due to improved norfloxacin salts was further supported by live-dead imaging.

CONCLUSION

Norfloxacin carboxylic acid salts can act as potential alternatives in terms of drug resensitization and reuse.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our study shows that carboxylic acid salts of norfloxacin could be effectively employed to treat both planktonic- and biofilm-based infections caused by select members of ESKAPE pathogens.

Exploring the Genome and Phenotype of Multi-Drug Resistant Klebsiella pneumoniae of Clinical Origin

Klebsiella pneumoniae is an important nosocomial pathogen with an extraordinary resistant phenotype due to a combination of acquired resistant-elements and efflux mechanisms. In this study a detailed molecular characterization of 11 K. pneumoniae isolates of clinical origin was carried out. Eleven clinical isolates were tested for their susceptibilities, by disk diffusion and broth microdilution and interpreted according to CLSI guidelines. Efflux activity was determined by measuring the extrusion of ethidium bromide and biofilm formation was assessed following static growth in Müeller-Hinton and minimal media M9 broths at two temperatures and time points. Template DNA from all 11 isolates was extracted and sequenced. The study collection was found to be resistant to several (extended-spectrum beta-lactam) ESBL-type compounds along with several (fluoro)quinolones (FQ). Resistance to tetracycline accounted for 55% of the study collection (n = 6) and three of the 11 isolates were resistance to carbapenems. Genotyping identified bla_CTX-M-15 (82%), bla_SHV-12 (55%), and bla_TEM-1B (45%) ESBL encoding genes and FQ resistance was associated the presence of the oqxAB operon, identified in 10 of the 11 isolates and qnrB gene in one isolate. The polymorphisms detected in the quinolone resistance-determining regions (QRDRs) were associated with isolates of the clonal group CG15. Sequence types (ST) identified were representative of previously described clonal groups including CG258 (n = 7), CG15 (n = 3), and CG147 (n = 1). Plasmid replicon type databases were queried indicating the presence of IncFII and IncFIB replicon types in the majority of the isolates (91%), followed by IncFIA (45%), and IncR (45%). Two of the 11 isolates were found positive for yersiniabactin siderophore-encoding genes. No differences in the ability to efflux ethidium bromide were identified. Biofilm formation was stronger when the isolates were grown under stressed conditions at 37°C for a period up to 96 h. These data confirm the fact that well-recognized clonal groups of K. pneumoniae of importance to human health carries a diverse repertoire of antimicrobial resistance determinants, particularly related to critically important drugs in the ESBL and FQ classes. The capacity of most isolates to form strong biofilms, when stressed under laboratory-simulated conditions, supports the risk to human health associated with nosocomial infections deriving from indwelling medical devices.

Molecular characterization of antimicrobial multi-drug resistance in non-typhoidal Salmonellae from chicken and clam in Mangalore, India

Salmonella enterica has been documented as one of the leading causes of salmonellosis throughout the world and is most commonly associated with the consumption of contaminated food products. Thus, this research was aimed at studying the antimicrobial susceptibility pattern and detection of quinolone resistance in Salmonella spp isolated from food of animal origin. Thirty-six Salmonella isolates comprising 8 from poultry and 28 from seafood (clams) were identified, serotyped and characterized for their antimicrobial susceptibility against 10 different antibiotics. Plasmid DNA was isolated from all the isolates by alkaline lysis, quinolone resistant non-typhoidal S.Weltevreden were examined for mutation in the DNA gyrase coding gene. Among the 36 Salmonella isolates, 20 were S. weltevreden (8 from poultry and 12 from seafood) and 16 were S.Typhimurium (from seafood). All the isolates showed multiple resistance to nalidixic acid, tetracycline, co-trimoxazole and nitrofurantoin, but, interestingly, the isolates were 100% susceptible to ampicillin, chloramphenicol and gentamicin. Resistant isolates from the study carried the genes responsible for resistance to respective antibiotics. The strain S130 isolated in the study showed single point mutation, Asp87Gly, at position 87 in quinolone resistance determining region. It revealed mutation in quinolone resistance determining region as a cause for quinolone resistance in non-typhoidal Salmonellae. The occurrence of genes accountable for plasmid mediated resistance to quinolones (viz., qnrA, qnrB and qnrS) in plasmid of non-typhoidal Salmonellae isolates provides evidence for plasmid mediated quinolone resistance.

Multiple Antimicrobial Resistance and Novel Point Mutation in Fluoroquinolone-Resistant Escherichia coli Isolates from Mangalore, India

Fluoroquinolone resistance in bacteria is usually associated with mutations in the topoisomerase regions. We report a novel point mutation in fluoroquinolone-resistant Escherichia coli strains. E. coli isolated from the environment in and around Mangalore, India, were examined for their antimicrobial resistance profile to 12 antibiotics and for the antibiotic resistance genes by polymerase chain reaction. Of the 67 E. coli isolated, 24 (35.8%) were sensitive to all antibiotics and 43 (64.2%) showed resistance to at least one of the 12 antibiotics used in the study. One isolate (EC10) was resistant to nine of the 12 antibiotics used. Resistance to nalidixic acid was the most common (34.32%), followed by nitrofurantoin (26.86%), tetracycline (22.38%), ampicillin (20.89%), cotrimoxazole (13.43%), ciprofloxacin (11.94%), gentamicin (10.44%), piperacillin/tazobactam (7.46%), chloramphenicol (7.46%), and cefotaxime (4.47%). Least resistance was observed for meropenem (1.49%) and none of the isolates showed resistance to imipenem. All the isolates harbored resistance genes corresponding to their antimicrobial resistance. Few quinolone-resistant isolates carried single point mutation (ser83Leu) and some had double point mutation (Ser83Leu and Asp87Asn) in gyrA. A third novel point mutation was also observed at position 50 with the change in the amino acid from tyrosine to cysteine (Tyr50Cys) in gyrA region. The study throws light on a novel point mutation in fluoroquinolone-resistant isolates. While the study helps to understand the risk and occurrence of antibiotic resistance among gram-negative bacteria from the environment, the alarming rate of antibiotic-resistant bacteria is a cause of concern in addressing infections.

Differential expression of virulence genes and role of gyrA mutations in quinolone resistant and susceptible strains of Salmonella Weltevreden and Newport isolated from seafood

AIMS

To investigate the differential expression of virulence genes and role of gyrA mutations in quinolone resistant and susceptible strains of Salmonella isolated from seafood.

METHODS AND RESULTS

Forty Salmonella isolates from seafood were tested for antibiotic sensitivity. Minimal inhibitory concentration (MIC) was determined and two nalidixic acid-resistant isolates, viz Salmonella Weltevreden (SW9) and Salmonella Newport (SN36) were selected for identifying the mechanism of resistance. SW9 showed mutation in the gyrA gene at codon 83 (Ser to Tyr) while SN36 presented at codon 87 (Asp to Asn). Experimental induction of resistance to a sensitive Salm. Newport (SN71) showed point mutation at codon 87 (Asp to Gly) in the gyrA gene, and was designated SN71R. All the isolates resistant to nalidixic acid had a single mutation at different positions in the gyrA gene. However, induction of resistance to a sensitive Salm. Weltevreden (SW30) was exceptional in that it did not show any mutation in the gyrA region. Use of Phe-Arg-β-naphthylamide (PAβN) also could not reduce MIC below the Clinical and Laboratory Standards Institute guidelines revealing the absence of efflux mediated resistance. Thus, the resistance mechanism in SW30R is unknown. The growth rate of quinolone resistant isolates was slower than the susceptible ones. The resistant isolates showed decreased epithelial cell invasion and intracellular replication. The mRNA expression levels of some of the genes were significantly (P < 0·005) reduced in SN71R compared to the sensitive strain (SN71).

CONCLUSIONS

Nalidixic acid-resistant Salmonella strains are associated with lower virulence and pathogenicity than the sensitive strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provided valuable information on the difference in the growth, cytotoxicity, infectivity and expression of virulence genes in resistant and susceptible strains. Furthermore, the gyrA mutation was shown to be the main mechanism of quinolone resistance in Salmonella other than the overexpression of efflux pumps or the presence of plasmid mediated quinolone resistance genes.

A Multicountry Molecular Analysis of Salmonella enterica Serovar Typhi With Reduced Susceptibility to Ciprofloxacin in Sub-Saharan Africa

BACKGROUND

Salmonella enterica serovar Typhi is a predominant cause of bloodstream infections in sub-Saharan Africa (SSA). Increasing numbers of S. Typhi with resistance to ciprofloxacin have been reported from different parts of the world. However, data from SSA are limited. In this study, we aimed to measure the ciprofloxacin susceptibility of S. Typhi isolated from patients with febrile illness in SSA.

METHODS

Febrile patients from 9 sites within 6 countries in SSA with a body temperature of ≥38.0°C were enrolled in this study. Blood samples were obtained for bacterial culture, and Salmonella isolates were identified biochemically and confirmed by multiplex polymerase chain reaction (PCR). Antimicrobial susceptibility of all Salmonella isolates was performed by disk diffusion test, and minimum inhibitory concentrations (MICs) against ciprofloxacin were measured by Etest. All Salmonella isolates with reduced susceptibility to ciprofloxacin (MIC > 0.06 µg/mL) were screened for mutations in quinolone resistance-determining regions in target genes, and the presence of plasmid-mediated quinolone resistance (PMQR) genes was assessed by PCR.

RESULTS

A total of 8161 blood cultures were performed, and 100 (1.2%) S. Typhi, 2 (<0.1%) Salmonella enterica serovar Paratyphi A, and 27 (0.3%) nontyphoid Salmonella (NTS) were isolated. Multidrug-resistant S. Typhi were isolated in Kenya (79% [n = 38]) and Tanzania (89% [n = 8]) only. Reduced ciprofloxacin-susceptible (22% [n = 11]) S. Typhi were isolated only in Kenya. Among those 11 isolates, all had a Glu133Gly mutation in the gyrA gene combined with either a gyrA (Ser83Phe) or gyrB mutation (Ser464Phe). One Salmonella Paratyphi A isolate with reduced susceptibility to ciprofloxacin was found in Senegal, with 1 mutation in gyrA (Ser83Phe) and a second mutation in parC (Ser57Phe). Mutations in the parE gene and PMQR genes were not detected in any isolate.

CONCLUSIONS

Salmonella Typhi with reduced susceptibility to ciprofloxacin was not distributed homogenously throughout SSA. Its prevalence was very high in Kenya, and was not observed in other study countries. Continuous monitoring of antimicrobial susceptibility is required to follow the potential spread of antimicrobial-resistant isolates throughout SSA.

Emergence of Ciprofloxacin-Resistant Salmonella enterica Serovar Typhi in Italy

In developed countries, typhoid fever is often associated with persons who travel to endemic areas or immigrate from them. Typhoid fever is a systemic infection caused by Salmonella enterica serovar Typhi. Because of the emergence of antimicrobial resistance to standard first-line drugs, fluoroquinolones are the drugs of choice. Resistance to ciprofloxacin by this Salmonella serovar represents an emerging public health issue. Two S. enterica ser. Typhi strains resistant to ciprofloxacin (CIP) were reported to the Italian surveillance system for foodborne and waterborne diseases (EnterNet-Italia) in 2013. The strains were isolated from two Italian tourists upon their arrival from India. A retrospective analysis of 17 other S. enterica ser. Typhi strains isolated in Italy during 2011–2013 was performed to determine their resistance to CIP. For this purpose, we assayed for susceptibility to antimicrobial agents and conducted PCR and nucleotide sequence analyses. Moreover, all strains were typed using pulsed-field gel electrophoresis to evaluate possible clonal relationships. Sixty-eight percent of the S. enterica ser. Typhi strains were resistant to CIP (MICs, 0.125–16 mg/L), and all isolates were negative for determinants of plasmid-mediated quinolone resistance. Analysis of sequences encoding DNA gyrase and topoisomerase IV subunits revealed mutations in gyrA, gyrB, and parC. Thirteen different clonal groups were detected, and the two CIP-resistant strains isolated from the individuals who visited India exhibited the same PFGE pattern. Because of these findings, the emergence of CIP-resistant S. enterica ser. Typhi isolates in Italy deserves attention, and monitoring antibiotic susceptibility is important for efficiently managing cases of typhoid fever.

Serotypes, seasonal trends, and antibiotic resistance of non-typhoidal Salmonella from human patients in Guangdong Province, China, 2009-2012

Background

Non-typhoidal Salmonella is a common cause of infectious diarrhea in humans. Antimicrobial-resistant Salmonella has become a global concern.

Methods

Using laboratory-based surveillance system for Salmonella from September 2009 to December 2012 in Guangdong Province of China. The clinical information and samples of diarrhea patients were collected, according to the surveillance case definition. The lab tests were followed by standardized protocols, including sample isolation, isolates confirmation, serotyping, and antimicrobial susceptibility testing (AST).

Results

A total of 1,826 Salmonella isolates were identified from40,572 patients in 28 hospitals in11 prefectures. The isolates ratio was highest in autumn (38.8%, 708/1826) and lowest in winter (6.4%, 117/1826). Children aged <5 years were the group most affected by Salmonella in Guangdong Province accounting for 73% (1,329/1,826), of whom the infants (<1 year) were 81.5% (1084/1329) especially. A total of 108 serotypes were identified among the isolates. S. Typhimurium represented the most common serotype followed by serotype 4,5,12:i:-.

S. Typhimurium was also the common serotype followed by S. Enteritidis among infants and children aged 1-3 years old. However, S. Enteritidis became the common serotype followed by S. Typhimurium among children aged 3–5 and >5 years.

Resistance to at least one antimicrobial was found in 72% (1321/1,826) of the isolates. Resistance to at least three antimicrobials was found in 46% (850/1,826) of the isolates. Resistance to all 12 antimicrobials screened was observed in 8 isolates (0.44%, 8/1,826). The resistant prevalence to quinolones including nalidixic acid and ciprofloxacin was 61.9% (1131/1826), of which ciprofloxacin resistance rate was 8.05% (147/1826). The prevalence resistance to all three cephalosporin antimicrobials (cefepime, cefotaxime, and caftazidime) in <5 yr age group was accounted for 90% (89/99).

Conclusions

Additional data and more refined methods can improve future surveillance. The invasive Salmonella isolates should also be included to the antibiotic resistance surveillance for clinical care or public health.

Mutant prevention concentrations of levofloxacin, pazufloxacin and ciprofloxacin for A. baumannii and mutations in gyrA and parC genes

Fluoroquinolones are antimicrobial agents that are widely used clinically, but the increasing resistance of Acinetobacter baumannii (A. baumannii) to these agents is a matter of concern. We investigated mutant prevention concentrations (MPCs) of three fluoroquinolones, levofloxacin (LVX), pazufloxacin (PAZ) and ciprofloxacin (CIP). We analyzed an A. baumannii standard strain (ATCC19606) for mutation prevention indices (MPIs), MPCs and mutant selection windows as well as MICs of CIP, PAZ and LVX and compared the derived values with 34 A. baumannii strains collected in hospitals. In addition, A. baumannii standard strain (ATCC19606) fluoroquinolone-resistant mutants were investigated for gyrA and parC gene mutations. MPCs of CIP, prevention antibiotics concentration and LVX for A. baumannii ATCC19606 were 12.8, 5.6 and 2.8 μg ml(-1) and their MPIs were 16, 8 and 4, respectively. Clinically isolated A. baumannii strains had CIP, PAZ and LVX MPC value ranges of 1-8, 1-16 and 0.5-2 μg ml(-1) and their MPIs were 8, 8 and 4 μg ml(-1). Single gyrA mutations (Ser(83)-Leu(83)) occurred in 18 resistant strains (48.7%) and single parC mutations (Ala(79)-Asp(79) or (Ser(80)-Leu(80)) occurred in 8 resistant strains (21.6%), whereas gyrA and parC double mutations occurred in 2 (5.4%) of the resistant strains. MPC and MPI values of LVX were lower than that of CIP and PAZ. Single gyrA and parC mutations accounted for the majority of mutations (n=24), whereas double mutations occurred only in two strains.

PMQR genes oqxAB and aac(6')Ib-cr accelerate the development of fluoroquinolone resistance in Salmonella typhimurium

Emergence of multidrug-resistant Salmonella typhimurium strains, especially the ACSSuT and nalidixic acid R types, has significantly compromised the effectiveness of current strategies to control Salmonella infections, resulting in increased morbidity and mortality. Clinical S. typhimurium isolates recovered in Hong Kong during the period of 2005–2011 were increasingly resistant to ciprofloxacin (CIP) and antibiotics of the ACSSuT group. Our data revealed that oqxAB and aac(6′)Ib-cr were encoded on plasmids of various sizes and the presence of these two elements together with a single gyrA mutation in S. typhimurium were sufficient to mediate resistance to CIP. Acquisition of the oqxAB and aac(6′)Ib-cr encoding plasmids by S. typhimurium caused a fourfold increase in CIP minimal inhibitory concentration. Furthermore, the presence of oqxAB and aac(6′)Ib-cr in Salmonella dramatically increased the mutation prevention concentration of CIP which may due to mutational changes in the drug target genes. In conclusion, possession of oqxAB and aac(6′)Ib-cr encoding plasmid facilitate the selection of CIP resistant S. typhimurium, thereby causing a remarkable increase of CIP resistance among clinical Salmonella strains in Hong Kong.

Prevalence, serovars, phage types, and antibiotic susceptibilities of Salmonella strains isolated from animals in the United Arab Emirates from 1996 to 2009

The aim of this study was to give some insights into the prevalence, serovars, phage types, and antibiotic resistances of Salmonella from animal origin in the United Arab Emirates. Data on diagnostic samples from animals (n = 20,871) examined for Salmonella between 1996 and 2009 were extracted from the databases of the Central Veterinary Research Laboratory in Dubai and from typed strains (n = 1052) from the Robert Koch Institute, Wernigerode Branch in Germany and analyzed for general and animal-specific trends. Salmonella was isolated from 1,928 (9 %) of the 20,871 samples examined. Among the 1,052 typed strains, most were from camels (n = 232), falcons (n = 166), bustards (n = 101), antelopes (n = 66), and horses (n = 63). The predominant serovars were Salmonella Typhimurium (25 %), Salmonella Kentucky (8 %), followed by Salmonella Frintrop (7 %), and Salmonella Hindmarsh (5 %). When analyzed by animal species, the most frequent serovars in camels were Salmonella Frintrop (28 %) and Salmonella Hindmarsh (21 %), in falcons Salmonella Typhimurium (32 %), in bustards Salmonella Kentucky (19 %), in antelopes Salmonella Typhimurium (9 %), and in horses Salmonella Typhimurium (17 %) and S. Kentucky (16 %). Resistance of all typed Salmonella strains (n = 1052) was most often seen to tetracycline (23 %), streptomycin (22 %), nalidixic acid (18 %), and ampicillin (15 %). These data show trends in the epidemiology of Salmonella in different animal species which can be used as a base for future prevention, control, and therapy strategies.

A multiplex single nucleotide polymorphism typing assay for detecting mutations that result in decreased fluoroquinolone susceptibility in Salmonella enterica serovars Typhi and Paratyphi A

OBJECTIVES

Decreased susceptibility to fluoroquinolones has become a major problem for the successful therapy of human infections caused by Salmonella enterica, especially the life-threatening typhoid and paratyphoid fevers.

METHODS

By using Luminex xTAG beads, we developed a rapid, reliable and cost-effective multiplexed genotyping assay for simultaneously detecting 11 mutations in gyrA, gyrB and parE of S. enterica serovars Typhi and Paratyphi A that result in nalidixic acid resistance (Nal(R)) and/or decreased susceptibility to fluoroquinolones.

RESULTS

This assay yielded unambiguous single nucleotide polymorphism calls on extracted DNA from 292 isolates of Salmonella Typhi (Nal(R) = 223 and Nal(S) = 69) and 106 isolates of Salmonella Paratyphi A (Nal(R) = 24 and Nal(S) = 82). All of the 247 Nal(R) Salmonella Typhi and Salmonella Paratyphi A isolates were found to harbour at least one of the target mutations, with GyrA Phe-83 as the most common one (143/223 for Salmonella Typhi and 18/24 for Salmonella Paratyphi A). We also identified three GyrB mutations in eight Nal(S) Salmonella Typhi isolates (six for GyrB Phe-464, one for GyrB Leu-465 and one for GyrB Asp-466), and mutations GyrB Phe-464 and GyrB Asp-466 seem to be related to the decreased ciprofloxacin susceptibility phenotype in Salmonella Typhi. This assay can also be used directly on boiled single colonies.

CONCLUSIONS

The assay presented here would be useful for clinical and reference laboratories to rapidly screen quinolone-resistant isolates of Salmonella Typhi and Salmonella Paratyphi A, and decipher the underlying genetic changes for epidemiological purposes.

Response of porcine intestinal in vitro organ culture tissues following exposure to Lactobacillus plantarum JC1 and Salmonella enterica serovar Typhimurium SL1344

The development of novel intervention strategies for the control of zoonoses caused by bacteria such as Salmonella spp. in livestock requires appropriate experimental models to assess their suitability. Here, a novel porcine intestinal in vitro organ culture (IVOC) model utilizing cell crown (CC) technology (CCIVOC) (Scaffdex) was developed. The CCIVOC model was employed to investigate the characteristics of association of S. enterica serovar Typhimurium strain SL1344 with porcine intestinal tissue following exposure to a Lactobacillus plantarum strain. The association of bacteria to host cells was examined by light microscopy and electron microscopy (EM) after appropriate treatments and staining, while changes in the proteome of porcine jejunal tissues were investigated using quantitative label-free proteomics. Exposure of porcine intestinal mucosal tissues to L. plantarum JC1 did not reduce the numbers of S. Typhimurium bacteria associating to the tissues but was associated with significant (P < 0.005) reductions in the percentages of areas of intestinal IVOC tissues giving positive staining results for acidic mucins. Conversely, the quantity of neutrally charged mucins present within the goblet cells of the IVOC tissues increased significantly (P < 0.05). In addition, tubulin-α was expressed at high levels following inoculation of jejunal IVOC tissues with L. plantarum. Although L. plantarum JC1 did not reduce the association of S. Typhimurium strain SL1344 to the jejunal IVOC tissues, detection of increased acidic mucin secretion, host cytoskeletal rearrangements, and proteins involved in the porcine immune response demonstrated that this strain of L. plantarum may contribute to protecting the pig from infections by S. Typhimurium or other pathogens.

Surveillance of antibiotic susceptibility patterns among Shigella sonnei strains isolated in Belgium during the 18-year period 1990 to 2007

This study was conducted to determine the frequency and pattern of antimicrobial susceptibility of Shigella sonnei, the predominant species causing shigellosis in Belgium. Between 1990 and 2007, a total of 7,307 strains, mainly (98.2%) isolated from stools, were diagnosed by peripheral laboratories before being confirmed as Shigella strains by serotyping by the National Reference Center of Salmonella and Shigella. A significant increase in resistances to tetracycline, streptomycin, trimethoprim, sulfonamides, and cotrimoxazole (i.e., trimethoprim in combination with sulfonamides) was observed during this period. Since 1998, resistance to nalidixic acid also increased to reach a peak (12.8%) of resistant isolates in 2004. Concomitantly, multidrug resistance (MDR) in this species emerged in 2007, with 82% of total isolates being MDR. However, during this 18-year period, all isolates remained fully susceptible to ciprofloxacin and gentamicin. The work includes the molecular characterization of mechanisms of resistance to ampicillin, tetracycline, chloramphenicol, and cotrimoxazole and class 1 and class 2 integrons. S. sonnei acquired antimicrobial resistance to traditional antibiotics (ampicillin and tetracycline) by horizontal gene transfer, while the genetic stability of transposons was responsible for a high (89%) proportion of resistance to a commonly prescribed antibiotic (cotrimoxazole). Therefore, cotrimoxazole should no longer be considered appropriate as empirical therapy for treatment of shigellosis in Belgium when antibiotics are indicated. Rates of resistance to nalidixic acid should also be attentively monitored to detect any shift in fluoroquinolone resistance, because it represents the first line among antibiotics used in the treatment of shigellosis.

Analysis of ciprofloxacin-resistant Salmonella strains from swine, chicken, and their carcasses in Taiwan and detection of parC resistance mutations by a mismatch amplification mutation assay PCR

One hundred thirty-three Salmonella strains isolated from the viscera of swine, chicken, and carcasses of swine and chicken in Taiwan from 2004 to 2006 were identified to serotype level and analyzed for their susceptibility to ciprofloxacin. In total, 76 (57%) strains of the Salmonella isolates exhibited high-level resistance to ciprofloxacin, having MICs ranging from 16 to 64 microg/ml. DNA sequence analysis revealed that mutations in the quinolone resistance-determining regions of gyrA (Ser83Phe, Asp87Gly or Asp87Asn), parC (Ser80Arg, or Ser80Ile or Glu84Lys), and parE (Ser458Pro) genes were associated with the Salmonella strains that demonstrated resistance to ciprofloxacin. A mismatch amplification mutation assay (MAMA)-PCR was developed to identify mutations in parC at codons 80 and 84. Specific PCR products were only recovered from ciprofloxacin-resistant Salmonella strains but not from the susceptible strains. MAMA-PCR targeting the mutations in parC correlated with what DNA sequencing revealed. In conclusion, monitoring ciprofloxacin-resistant Salmonella from animal sources should be performed on a regular basis. MAMA-PCR targeting parC could provide a fast method for those laboratories interested in quickly characterizing the resistance profile and with little access to DNA sequencing facilities.

Genetic variability among ampC genes from acinetobacter genomic species 3

As a part of a nationwide study in Spain, 15 clinical isolates of Acinetobacter genomic species 3 (AG3) were analyzed. The main objective of the study was to characterize the ampC genes from these isolates and to determine their involvement in beta-lactam resistance in AG3. The 15 AG3 isolates showed different profiles of resistance to ampicillin (range of MICs, 12 to >256 microg/ml). Nucleotide sequencing of the 15 ampC genes yielded 12 new AmpC enzymes (ADC-12 to ADC-23). The 12 AG3 enzymes showed 93.7 to 96.1% amino acid identity with respect to the AmpC enzyme from Acinetobacter baumannii (ADC-1 enzyme). Eight out of fifteen ampC genes were expressed in Escherichia coli cells under the control of a common promoter, and with the exception of one isolate (isolate 65, which showed lower beta-lactam MICs), significant differences in overall beta-lactam MICs for E. coli cells expressing AG3 ampC genes were not revealed. No significant differences in ampC gene expression in AG3 clinical isolates were revealed by reverse transcription-PCR analysis. A detailed analysis of the 12 AmpC protein sequences revealed that amino acid replacements (in comparison with those of ADC-1) occurred mainly in the same positions, although none were located in important functional domains such as the Omega- loop or conserved beta-lactamase motifs. Kinetic experiments performed with three representative AmpC enzymes (ADC-14, -16, and -18) in some cases revealed dramatic changes in K(m) and k(cat) values for beta-lactams. No ISAba1 was detected upstream of the ampC genes. Our results reveal 12 new ampC genes in AG3. The enzymes showed a moderate degree of variability, and they are tentatively named ADC-12 to ADC-23.

International spread of multidrug-resistant Salmonella Schwarzengrund in food products

This serovar was isolated from persons, food, and food animals in Thailand, Denmark, and the United States.

We compared 581 Salmonella enterica serotype Schwarzengrund isolates from persons, food, and food animals in Denmark, Thailand, and the United States by antimicrobial drug susceptibility and pulsed-field gel electrophoresis (PFGE) typing. Resistance, including resistance to nalidixic acid, was frequent among isolates from persons and chickens in Thailand, persons in the United States, and food imported from Thailand to Denmark and the United States. A total of 183 PFGE patterns were observed, and 136 (23.4%) isolates had the 3 most common patterns. Seven of 14 isolates from persons in Denmark had patterns found in persons and chicken meat in Thailand; 22 of 390 human isolates from the United States had patterns found in Denmark and Thailand. This study suggests spread of multidrug-resistant S. Schwarzengrund from chickens to persons in Thailand, and from imported Thai food products to persons in Denmark and the United States.

Clonal expansion and microevolution of quinolone-resistant Salmonella enterica serotype typhi in Vietnam from 1996 to 2004

Salmonella enterica serotype Typhi clinical isolates (n = 91) resistant to nalidixic acid (Nal(r)) were collected from sporadic cases and minor outbreaks throughout Vietnam between 1996 and 2004. These isolates were typed and compared by four methods: Vi phage typing, PstI ribotyping, XbaI and SpeI pulsed-field gel electrophoresis (PFGE), and single-nucleotide polymorphism (SNP) analysis. The results indicated that 65% of the isolates were not typeable by Vi phage typing. In contrast, the ribotyping and, with more accuracy, the SNP analysis methods indicated that all Nal(r) isolates belonged to a single clone (ribotype 3a, haplotype H58) that was found previously and that largely consisted of plasmid-encoded multidrug-resistant serotype Typhi isolates. PFGE demonstrated the occurrence of microevolution within this clone. We identified two major combined PFGE profiles: X1-S1 and X3-S6. X3-S6 predominated between 1996 and 2002 but was replaced by X1-S1 after 2002. Nevertheless, PFGE, with a Simpson's index of 0.78, was not considered an optimal discriminatory method for investigating typhoid fever outbreaks in Vietnam. The rate of quinolone resistance increased and the rate of multidrug resistance decreased during the study period. From 2002 to 2004, 80.6% of the isolates from South Vietnam were resistant only to Nal. The mechanism of Nal resistance in most of the isolates (94%) was a mutation in the quinolone resistance-determining chromosomal region of gyrA that led to the amino acid substitution Ser83Phe. No plasmid-located qnrA, qnrB, or qnrS was detected.

Quinolone resistance of Salmonella enterica serovar Virchow isolates from humans and poultry in Israel: evidence for clonal expansion

Salmonella enterica serovar Virchow is highly prevalent in humans and farm animals in Israel. In addition to high rates of resistance to multiple antibiotics, this serovar exhibits a high incidence of resistance to nalidixic acid. More than 90% of Salmonella serovar Virchow isolates of human and poultry origin obtained from 1997 to 2004 were resistant to nalidixic acid (MIC > or = 128 microg/ml), with reduced susceptibility to ciprofloxacin (MIC between 0.125 and 0.250 microg/ml). Most isolates belonged to two predominant, closely related pulsed-field gel electrophoresis image types. Investigation of the mechanisms of quinolone resistance revealed that this pathogen probably emerged from a parental clone that overproduced the AcrAB efflux pump and had a single point mutation in gyrA leading to the Asp87Tyr substitution. The close resemblance between human and poultry isolates points to poultry as a likely source of Salmonella serovar Virchow in the food chain.

Antimicrobial resistance in nontyphoidal Salmonella

Salmonella is one of the leading causes of foodborne illness in countries around the world. Treatment of Salmonella infections, in both animals and humans has become more difficult with the emergence of multidrug-resistant (MDR) Salmonella strains. Foodborne infections and outbreaks with MDR Salmonella are also increasingly reported. To better monitor and control the spread of MDR Salmonella, it is important to understand the mechanisms responsible for drug resistance and how drug resistance is transmitted to and between Salmonella strains. This review summarizes current knowledge on antimicrobial drugs used to treat Salmonella infections and provides an overview of MDR Salmonella in the United States and a discussion of the genetics of Salmonella drug resistance, including the mechanisms responsible for the transmission of drug-resistance genes in Salmonella, using data from the United States and other countries.

Efflux pumps as antimicrobial resistance mechanisms

Antibiotic resistance continues to hamper antimicrobial chemotherapy of infectious disease, and while biocide resistance outside of the laboratory is as yet unrealized, in vitro and in vivo episodes of reduced biocide susceptibility are not uncommon. Efflux mechanisms, both drug-specific and multidrug, are important determinants of intrinsic and/or acquired resistance to these antimicrobials in important human pathogens. Multidrug efflux mechanisms are generally chromosome-encoded, with their expression typically resultant from mutations in regulatory genes, while drug-specific efflux mechanisms are encoded by mobile genetic elements whose acquisition is sufficient for resistance. While it has been suggested that drug-specific efflux systems originated from efflux determinants of self-protection in antibiotic-producing Actinomycetes, chromosomal multidrug efflux determinants, at least in Gram-negative bacteria, are appreciated as having an intended housekeeping function unrelated to drug export and resistance. Thus, it will be important to elucidate the intended natural function of these efflux mechanisms in order, for example, to anticipate environmental conditions or circumstances that might promote their expression and, so, compromise antimicrobial chemotherapy. Given the clinical significance of antimicrobial exporters, it is clear that efflux must be considered in formulating strategies for treatment of drug-resistant infections, both in the development of new agents, for example, less impacted by efflux or in targeting efflux directly with efflux inhibitors.

Clonal emergence of extended-spectrum beta-lactamase (CTX-M-2)-producing Salmonella enterica serovar Virchow isolates with reduced susceptibilities to ciprofloxacin among poultry and humans in Belgium and France (2000 to 2003)

Antibiotic treatment is not required in cases of Salmonella enterica gastroenteritis but is essential in cases of enteric fever or invasive salmonellosis or in immunocompromised patients. Although fluoroquinolones and extended-spectrum cephalosporins are the drugs of choice to treat invasive Salmonella, resistance to these antibiotics is increasing worldwide. During the period 2000 to 2003, 90 Salmonella enterica serovar Virchow poultry and poultry product isolates and 11 serovar Virchow human isolates were found to produce an extended-spectrum beta-lactamase, CTX-M-2, concomitantly with a TEM-1 beta-lactamase. The bla(CTX-M-2) gene was located on a large conjugative plasmid (>100 kb). Pulsed-field gel electrophoresis indicated a clonal relationship between the poultry and human isolates. All these isolates displayed additional resistance to trimethoprim-sulfamethoxazole and tetracycline as well as a reduced susceptibility to ciprofloxacin (MICs of between 0.5 and 1 mug/ml). CTX-M-2-producing Salmonella with a reduced susceptibility to fluoroquinolones constitutes a major concern, since such strains could disseminate on a large scale and jeopardize classical antibiotic therapy in immunocompromised patients.

Effect of exposure to fluoroquinolones and beta-lactams on the in vitro activity of other groups of antibiotics in Salmonella spp

After designing in vitro models of repeated exposure of Salmonella spp. to various beta-lactams and fluoroquinolones we studied the decrease in susceptibility to other antibiotic families of the mutants generated. There was a decrease in the susceptibility of all the mutants to tetracycline, cotrimoxazole and chloramphenicol. Mutants generated following exposure to fluoroquinolones showed reduced susceptibility to amoxicillin, amoxicillin/clavulanic acid, cefoxitin and cefuroxime, whereas mutants generated following beta-lactam exposure showed reduced susceptibility to nalidixic acid and ciprofloxacin. We observed that the efflux pump systems are activated in the mutants generated and this may therefore be the cause of the decrease in susceptibility. In many cases the decrease is small and is not detected if the CLSI criteria are applied. Nevertheless, more detailed studies should be done to evaluate the importance of this phenomenon and rationalize the use of antibiotics in both humans and animals so as to control the increase in the number of multiresistant strains.

Multidrug resistance in Salmonella enterica serotype Typhimurium from humans in France (1993 to 2003)

The aim of this study was to determine the distribution of the antimicrobial resistance phenotypes (R types), the phage types and XbaI-pulsed-field gel electrophoresis (PFGE) types, the genes coding for resistance to beta-lactams and to quinolones, and the class 1 integrons among a representative sample of Salmonella enterica serotype Typhimurium isolates collected from humans in 2002 through the French National Reference Center for Salmonella (NRC-Salm) network. The trends in the evolution of antimicrobial resistance of serotype Typhimurium were reviewed by using NRC-Salm data from 1993, 1997, 2000, and 2003. In 2002, 3,998 isolates of serotype Typhimurium were registered at the NRC-Salm among 11,775 serotyped S. enterica isolates (34%). The most common multiple antibiotic resistance pattern was resistance to amoxicillin, chloramphenicol, streptomycin and spectinomycin, sulfonamides, and tetracycline (ACSSpSuTe R type), with 156 isolates (48.8%). One isolate resistant to extended-spectrum cephalosporins due to the production of TEM-52 extended-spectrum beta-lactamase was detected (0.3%), and one multidrug-resistant isolate was highly resistant to ciprofloxacin (MIC > 32 mg/liter). We found that 57.2% of the isolates tested belonged to the DT104 clone. The main resistance pattern of DT104 isolates was R type ACSSpSuTe (83.2%). However, evolutionary changes have occurred within DT104, involving both loss (variants of Salmonella genomic island 1) and acquisition of genes for drug resistance to trimethoprim or to quinolones. PFGE profile X1 was the most prevalent (74.5%) among DT104 isolates, indicating the need to use a more discriminatory subtyping method for such isolates. Global data from the NRC-Salm suggested that DT104 was the main cause of multidrug resistance in serotype Typhimurium from humans from at least 1997 to 2003, with a roughly stable prevalence during this period.

Genes and mutations conferring antimicrobial resistance in Salmonella: an update

Resistance to various classes of antimicrobial agents has been encountered in many bacteria of medical and veterinary relevance. Particular attention has been paid to zoonotic bacteria such as Salmonella. Over the years, various studies have reported the presence of genes and mutations conferring resistance to antimicrobial agents in Salmonella isolates. This review is intended to provide an update on what is currently known about the genetic basis of antimicrobial resistance in Salmonella.

Resistance to fluoroquinolones in Salmonella: emerging mechanisms and resistance prevention strategies

We review the current state of knowledge about the genetic and biochemical mechanisms that mediate quinolone resistance in Salmonella. They include modifications of topoisomerase targets, increased efflux activity and the recently described topoisomerase protection by the plasmid-encoded Qnr protein. We discuss what factors may determine the order of implementation of these various mechanisms in a particular strain, and what strategies could be used to combat resistance, from the inhibition of mutagenesis mechanisms to counteracting, during fluoroquinolone treatment, of resistance mechanisms already set in the infecting strain.

Ciprofloxacin-resistant Salmonella enterica serovar Typhimurium strains are difficult to select in the absence of AcrB and TolC

It has been proposed that lack of a functional efflux system(s) will lead to a lower frequency of selection of resistance to fluoroquinolones and other antibiotics. We constructed five strains of Salmonella enterica serovar Typhimurium SL1344 that lacked efflux gene components of resistance nodulation cell division pumps (acrB, acrD, acrF, acrBacrF, and tolC) plus three strains that lack genes that effect efflux gene expression (marA, soxS, and ramA) and a hypermutable strain (mutS::aph). Strains were exposed to ciprofloxacin at 2x the MIC in agar, in the presence and absence of Phe-Arg-beta-naphthylamide, an efflux pump inhibitor. Mutants were selected from all strains except those lacking acrB, tolC, or acrBacrF. For strains from which mutants were selected, there were no significant differences between the frequencies of resistance. Except for mutants of the ramA::aph strain, two phenotypes arose: resistance to quinolones only and multiple antibiotic resistance (MAR). ramA::aph mutants were resistant to quinolones only, suggesting a role for ramA in MAR in S. enterica serovar Typhimurium. Phe-Arg-beta-naphthylamide (20 microg/ml) had no effect on the frequencies of resistance or ciprofloxacin MICs. In conclusion, functional AcrB and TolC in S. enterica serovar Typhimurium are important for the selection of ciprofloxacin-resistant mutants.

Analysis of Salmonella spp. with resistance to extended-spectrum cephalosporins and fluoroquinolones isolated in North America and Latin America: report from the SENTRY Antimicrobial Surveillance Program (1997–2004)

Emerging antimicrobial-resistant Salmonella spp. requires increased efforts to appropriately test susceptibility. The SENTRY Antimicrobial Surveillance Program monitored Salmonella spp. and detected nalidixic acid-resistant strains with elevated fluoroquinolone minimum inhibitory concentration (MIC) results and strains with extended-spectrum beta-lactamase (ESBL) "phenotypes" over the last 8 years. A total of 786 stool and bloodstream isolates from North American and Latin American medical centers (2001-2003) were tested by reference broth microdilution methods. Genetic analysis was used to further characterize the resistance mechanisms. Twenty-one sites forwarded 89 (11.3%) nalidixic acid-resistant (MIC, > or =32 microg/mL) strains. Nineteen of these isolates were studied to determine mutations in the quinolone resistance-determining region (QRDR). Among the nalidixic acid-resistant Salmonella spp. isolates, fluoroquinolone MIC values were also elevated (8- to 32-fold) compared with "wild-type" strains. Ciprofloxacin and gatifloxacin (MIC(90), 0.5 microg/mL) were more potent than levofloxacin and garenoxacin (1 microg/mL) against nalidixic acid-resistant strains. Single gyrA mutations were responsible for elevated fluoroquinolone MIC values and included D87Y (5), S83F (7), D87N (5), and S83Y (2). During 2001, 9 sites contributed 11 (2.9%) strains that met ESBL screening criteria (> or =2 microg/mL) for aztreonam or ceftazidime or ceftriaxone. ESBL confirmation was evaluated by Etest (AB BIODISK, Solna, Sweden) ESBL strips and the enzymes were characterized by polymerase chain reaction and gene sequencing. The ESBL phenotype isolates had the following MIC patterns: ceftazidime (> or =16 microg/mL), aztreonam (4 to >16 microg/mL), and ceftriaxone (8-32 microg/mL). All strains were susceptible to cefepime, carbapenems, gentamicin, and fluoroquinolones. No strains were inhibited by clavulanic acid consistent with all isolates producing the identified CMY-2, AmpC-like enzyme. Fluoroquinolones may be compromised among isolates with QRDR mutations detected using nalidixic acid as a screening agent. Salmonella spp. with ESBL phenotypes were likely to harbor CMY-2 (not an ESBL) and remain susceptible to cefepime, carbapenems, and fluoroquinolones, which can be used for serious invasive Salmonella spp. infections. Compared with the stool culture isolates, the blood culture isolates had higher QRDR mutations, but remained susceptible to the fluoroquinolones. The blood culture isolates were more susceptible to penicillins (ampicillin and ticarcillin) and not significantly different for ceftriaxone or trimethoprim/sulfamethoxazole susceptibility patterns. No QRDR trends over time were detected in North America, but increased resistance was observed in Latin America.

Recent trends in the epidemiology of non-typhoid Salmonella and antimicrobial resistance: the Israeli experience and worldwide review

PURPOSE OF REVIEW

The epidemiology of non-typhoid Salmonella has changed significantly since the turn of the century. Interestingly, non-typhoid Salmonella epidemiology in Israel mirrors some important global trends, and these new trends are reviewed. Recent research that has shed more light on the true toll of non-typhoid Salmonella epidemic and resistance is also summarized.

RECENT FINDINGS

After more than three decades of a persistent rise, reports from Israel, the US, and the UK indicate that the trend may be reversed and the incidence of NTS illnesses is starting to decline. In contrast, the rates of resistance and multidrug resistance are increasing and expanding worldwide. Of major concern are the increasing rates of multidrug resistance in Salmonella typhimurium, particularly definitive phage-type 104, the alarming increase in low-level ciprofloxacin resistance among several non-typhoid Salmonella serotypes, and the upsurge of high-level ciprofloxacin resistance, mainly in Taiwan. In Israel, high rates of resistance were reported for Salmonella virchow, which accounts for 16% of non-typhoid Salmonella illnesses, and is highly invasive in children. The true burden of Salmonella illnesses in the US was calculated as 520 cases per 100 000, compared with an annual incidence of 13.4 per 100 000 of laboratory confirmed cases. Hospitalization and death rates were 20% and 0.6%, respectively. Infection with resistant non-typhoid Salmonella isolates, and particularly S. typhimurium, increases the likelihood of hospitalization and death.

SUMMARY

Many important trends of non-typhoid Salmonella epidemiology are not restricted to a single geographic location, but spread worldwide, reflecting the global nature of the epidemic. This epidemic imposes a heavy burden worldwide.

Characterization of isolates of Salmonella enterica serovar typhimurium displaying high-level fluoroquinolone resistance in Japan

Strains of the multidrug-resistant (MDR) Salmonella enterica serovar Typhimurium isolated in Japan were examined for high-level fluoroquinolone resistance. Since the first isolation in 2000 (described in reference 13), we have identified 12 human and 5 nonhuman isolates with high-level fluoroquinolone-resistance (ciprofloxacin MIC of 24 microg/ml or more). Most of these isolates shared some features including definitive phage type (DT 12/193), resistance type (ACSSuTNCp; resistant to ampicillin, chloramphenicol, streptomycin, sulfonamides, tetracycline, nalidixic acid, and ciprofloxacin), and genotype on pulsed-field gel electrophoresis that were different from those of the MDR S. enterica Typhimurium DT 104. Mutations in quinolone resistance-determining regions of gyrA and parC were also conserved in almost all of the isolates despite the absence of any apparent epidemiological relationships among cases. This suggests that a specific clonal group of the serovar Typhimurium with high levels of fluoroquinolone resistance is disseminating among animals and humans in Japan.

Clindamycin and quinolone therapy for Bacillus anthracis Sterne infection in 60Co-gamma-photon-irradiated and sham-irradiated mice

OBJECTIVES

Sublethal ionizing doses of radiation increase the susceptibility of mice to Bacillus anthracis Sterne infection. In this study, we investigated the efficacy of clindamycin in 60Co-gamma-photon-irradiated and sham-irradiated mice after intratracheal challenge with B. anthracis Sterne spores. Clindamycin has in vitro activity against B. anthracis and inhibits the production of toxin from other species, although no direct evidence exists that production of B. anthracis toxin is inhibited.

METHODS

Ten-week-old B6D2F1/J female mice were either sham-irradiated or given a sublethal 7 Gy dose of 60Co-gamma-photon radiation 4 days prior to an intratracheal challenge with toxigenic B. anthracis Sterne spores. Mice were treated twice daily with 200 mg/kg clindamycin (subcutaneous or oral), 100 mg/kg moxifloxacin (oral), 50 mg/kg ciprofloxacin (subcutaneous) or a combination therapy (clindamycin + ciprofloxacin). Bacteria were isolated and identified from lung, liver and heart blood at five timed intervals after irradiation. Survival was recorded twice daily following intratracheal challenge.

RESULTS

The use of clindamycin increased survival in gamma-irradiated and sham-irradiated animals challenged with B. anthracis Sterne in comparison with control mice (P < 0.001). Ciprofloxacin-treated animals had higher survival compared with clindamycin-treated animals in two experiments, and less survival in a third experiment, although differences were not statistically significant. Moxifloxacin was just as effective as clindamycin. Combination therapy did not improve survival of sham-irradiated animals and significantly decreased survival among gamma-irradiated animals (P = 0.01) in comparison with clindamycin-treated animals. B. anthracis Sterne was isolated from lung, liver and heart blood, irrespective of the antimicrobial treatment.

CONCLUSIONS

Treatment with clindamycin, ciprofloxacin or moxifloxacin increased survival in sham-irradiated and gamma-irradiated animals challenged intratracheally with B. anthracis Sterne spores. However, the combination of clindamycin and ciprofloxacin increased mortality associated with B. anthracis Sterne infection, particularly in gamma-irradiated animals.

Ciprofloxacin resistance in non-typhoidal Salmonella serotypes in Scotland, 1993–2003

OBJECTIVES

To determine the levels of resistance to ciprofloxacin among non-typhoidal salmonellae in Scotland.

METHODS

Etest strips were used to determine the ciprofloxacin MICs for nalidixic acid-resistant salmonellae isolated from humans during 1993-2003.

RESULTS

The percentage of nalidixic acid-resistant salmonellae isolated from human infection in Scotland has increased from 1.4% in 1993, to 8.8% in 1998 and 21.3% in 2003. Of 493 strains of nalidixic acid-resistant Salmonella tested, a single isolate of Salmonella Typhimurium RDNC was resistant to ciprofloxacin at the NCCLS breakpoint of 4 mg/L. Eleven isolates were resistant at the Scottish Salmonella Reference Laboratory breakpoint of 0.5 mg/L, while 466 isolates (94.5%) had reduced susceptibility with MICs of > or =0.125 mg/L and <0.5 mg/L.

CONCLUSIONS

These results show that very few non-typhoidal isolates of Salmonella in Scotland that are resistant to nalidixic acid are also resistant to ciprofloxacin at the NCCLS recommended breakpoint of 4 mg/L. The majority of isolates examined, however, do show reduced susceptibility at 0.125 mg/L. This confirms the necessity for continuing antimicrobial surveillance of resistance to this important antibiotic.

Efflux-mediated antimicrobial resistance

Antibiotic resistance continues to plague antimicrobial chemotherapy of infectious disease. And while true biocide resistance is as yet unrealized, in vitro and in vivo episodes of reduced biocide susceptibility are common and the history of antibiotic resistance should not be ignored in the development and use of biocidal agents. Efflux mechanisms of resistance, both drug specific and multidrug, are important determinants of intrinsic and/or acquired resistance to these antimicrobials, with some accommodating both antibiotics and biocides. This latter raises the spectre (as yet generally unrealized) of biocide selection of multiple antibiotic-resistant organisms. Multidrug efflux mechanisms are broadly conserved in bacteria, are almost invariably chromosome-encoded and their expression in many instances results from mutations in regulatory genes. In contrast, drug-specific efflux mechanisms are generally encoded by plasmids and/or other mobile genetic elements (transposons, integrons) that carry additional resistance genes, and so their ready acquisition is compounded by their association with multidrug resistance. While there is some support for the latter efflux systems arising from efflux determinants of self-protection in antibiotic-producing Streptomyces spp. and, thus, intended as drug exporters, increasingly, chromosomal multidrug efflux determinants, at least in Gram-negative bacteria, appear not to be intended as drug exporters but as exporters with, perhaps, a variety of other roles in bacterial cells. Still, given the clinical significance of multidrug (and drug-specific) exporters, efflux must be considered in formulating strategies/approaches to treating drug-resistant infections, both in the development of new agents, for example, less impacted by efflux and in targeting efflux directly with efflux inhibitors.

Mechanisms of quinolone resistance in Escherichia coli and Salmonella: Recent developments

Fluoroquinolones are broad-spectrum antimicrobials highly effective for treatment of a variety of clinical and veterinary infections. Their antibacterial activity is due to inhibition of DNA replication. Usually resistance arises spontaneously due to point mutations that result in amino acid substitutions within the topoisomerase subunits GyrA, GyrB, ParC or ParE, decreased expression of outer membrane porins, or overexpression of multidrug efflux pumps. In addition, the recent discovery of plasmid-mediated quinolone resistance could result in horizontal transfer of fluoroquinolone resistance between strains. Acquisition of high-level resistance appears to be a multifactorial process. Care needs to taken to avoid overuse of this important class of antimicrobial in both human and veterinary medicine to prevent an increase in the occurrence of resistant zoonotic and non-zoonotic bacterial pathogens that could subsequently cause human or animal infections.