Rapid Emergence and Clonal Dissemination of CTX-M-15-Producing Salmonella enterica Serotype Virchow, South Korea

Antibiotic Resistance Profiles and Molecular Characteristics of blaCMY-2-Carrying Salmonella enterica Serovar Albany Isolated from Chickens During 2013-2020 in South Korea

The production of β-lactamase by nontyphoidal Salmonella has become a public health issue throughout the world. In this study, we aimed to investigate the antimicrobial resistance profiles and molecular characteristics of β-lactamase-producing Salmonella enterica serovar Albany isolates. A total of 434 Salmonella Albany were obtained from feces and carcasses of healthy and diseased food-producing animals [cattle (n = 2), pigs (n = 3), chickens (n = 391), and ducks (n = 38)] during 2013-2020. Among the 434 Salmonella Albany isolates, 3.7% showed resistance to cefoxitin, and all the cefoxitin-resistant isolates were obtained from chickens. Moreover, Salmonella Albany isolates demonstrated high resistance to nalidixic acid (99.3%), trimethoprim/sulfamethoxazole (97.9%), ampicillin (86.6%), chloramphenicol (86.6%), and tetracycline (85.7%), as well as higher rates of multidrug resistance were detected in cefoxitin-resistant isolates compared to cefoxitin-susceptible isolates. All cefoxitin-resistant isolates harbored CMY-2-type β-lactamase and belonged to seven different pulsotypes, with type IV-b (43.75%) and IV-a (25%) making up the majority. In addition, genes encoding cefoxitin resistant of all blaCMY-2-harboring Salmonella Albany isolates were horizontally transmitted to a recipient Escherichia coli J53 by conjugation. Furthermore, 93.75% (15/16) of conjugative plasmids harboring blaCMY-2 genes belong to ST12/CC12-IncI1. Genetic characteristics of transmitted blaCMY-2 genes were associated with ISEcp1, which can play an essential role in the effective mobilization and expression of these genes. Salmonella Albany containing blaCMY-2 in chickens can potentially be transferred to humans. Therefore, it is necessary to restrict antibiotic use and conduct continuous monitoring and analysis of resistant bacteria in the poultry industry.

Molecular characterization of extended-spectrum cephalosporin and fluoroquinolone resistance genes in Salmonella and Shigella isolated from clinical specimens in Thailand

Antimicrobial resistance of Salmonella and Shigella has become a major clinical and public health problem. The incident of co-resistance to third generation cephalosporins and fluoroquinolone is a serious therapeutic issue in Thailand. The present study aimed to investigate the antimicrobial resistance and molecular character of clinical Shigella and Salmonella isolates. A total of 33 Salmonella and 53 Shigella cefotaxime-resistant isolates were collected from human clinical cases in Thailand during the period from 2011-2018. The antimicrobial susceptibility of Salmonella and Shigella was determined by the disk diffusion method, and extended-spectrum beta-lactamase (ESBL) production was characterized by the double-disk synergy test. Genotype characterization was performed by PCR and DNA sequencing. Thirty-two (97.0%) and fifty-two (98.1%) isolates of cefotaxime-resistant Salmonella and Shigella, respectively, were identified as ESBL producers. Shigella sonnei (4 isolates), Salmonella serovar 4,5,12:i:- (6 isolates), Salmonella serovar Agona (2 isolates) and Salmonella serovar Rissen (2 isolates) showed co-resistance to ciprofloxacin and cefotaxime or ceftriaxone. The combination of bla _CTX-M-15 plus other ESBL and/or AmpC β-lactamase genes was the most dominant of the genotype patterns in ESBL-producing isolates. The plasmid harbouring the aac(6')-Ib-cr gene and mutations of gyrA (S83F, D87Y or D87G) and parC (T57S) genes was found in 2 ESBL-producing Salmonella isolates. Three Shigella sonnei isolates harboured mutations in gyrA (S83L, D87Y or D87G), and only one Shigella sonnei phase I isolate showed mutations in both gyrA (S83L and D87G) and parC (S80I) genes. Among these clinical Shigella sonnei isolates, qnrS determinants were identified. Production of ESBLs is an important mechanism for resistance to extended-spectrum cephalosporins in Salmonella and Shigella. The emergence of a decreased susceptibility to extended-spectrum cephalosporins and fluoroquinolone in ESBL-producing isolates has important clinical and therapeutic implications.

Discovery and characterization of a new genotype of Salmonella enterica serovar Bareilly isolated from diarrhea patients of food-borne outbreaks

Since the first food-borne outbreak of Salmonella enterica serovar Bareilly in the UK (2010), it has been recognized as a new type of food-borne pathogen in S. enterica. To detect and characterize this new serovar pathogen in South Korea, a total of 175 Salmonella strains was isolated and 31 isolates were identified as S. Bareilly from various food-borne outbreaks between 2014 and 2018. While pulsed-field gel electrophoresis (PFGE) analysis using XbaI revealed two major groups (A and B) each with two subgroups (A1, A2/B1, B2), average nucleotide identity (ANI), single nucleotide polymorphism (SNP), and in silico multilocus sequence typing (MLST) analyses confirmed only two major groups. Interestingly, extended SNP analysis with 67 S. Bareilly strains from outbreaks in other countries revealed that A group strains between 2014 and 2016 shared a close evolutionary relationship with the strains from outside of South Korea; however, the B group strains in 2018 were located in a separate SNP tree branch. These findings suggest that the A group may share common ancestor with the strains of previous outbreaks in the UK or other countries, while the B group is a new genotype. Comparative virulence factor (VF) analysis between the A and B group strains showed that S. Bareilly in the B group has more various than that of the A group. A comparative biofilm formation assay supports for this, which B group strain GG-21 has higher biofilm formation activity than A group strain GG-07. Antibiotic susceptibility test of 31 S. Bareilly strains revealed high susceptibility to 17 tested antibiotics, suggesting that S. Bareilly can be easily treated by antibiotics.

Antibiotic Resistance Profiles and Molecular Characteristics of blaCTX-M-15-Carrying Salmonella enterica Serovar Enteritidis Isolates from Healthy and Diseased Chickens in Korea

Extended-spectrum β-lactamase (ESBL)-producing Salmonella enterica serovar Enteritidis has emerged as a public health concern. The main objectives of this study were therefore to determine the antimicrobial susceptibility profiles of Salmonella Enteritidis and to investigate the molecular characteristics of identified ESBL-producing isolates. In the study, 237 Salmonella Enteritidis isolates (232 isolates from chickens, 4 from cattle, and 1 from a pig) were recovered from carcasses and fecal samples of healthy and diseased food animals between 2010 and 2017. Ceftiofur resistance was noted only in chicken isolates (43%, 102/237), with the highest in healthy chickens and their carcasses (48.3%, 83/172) compared with that in diseased chickens (31.7%, 19/60). All of the ceftiofur-resistant isolates exhibited resistance to multiple antimicrobials. Indeed, a relatively higher percentage of ceftiofur-resistant isolates demonstrated resistance to the tested aminoglycosides and tetracycline compared with the ceftiofur-susceptible strains. In this study, bla_CTX-M-15 was the only ESBL gene detected in all of the ceftiofur-resistant isolates. The bla_CTX-M-15-carrying isolates belonged to 11 different pulsotypes. The bla_CTX-M-15 gene was transferred from 20.6% (21/102) of the bla_CTX-M-15-harboring isolates to a recipient Escherichia coli J53. The coexistence of IncHI2/ST2 and IncFIIs/ST1 plasmids was noted in the majority (81.8%, 18/22) of the transconjugants. E. coli J53 transconjugants carrying bla_CTX-M-15 gene showed distinct genetic environments, predominantly ISEcp1-bla_CTX-M-15-orf477 (15/21, 71.4%). This study demonstrated that healthy chickens and their carcasses act as reservoirs of bla_CTX-M-15-carrying Salmonella Enteritidis that can potentially be transmitted to humans.

Trends in ESBLs and PABLs among enteric Salmonella isolates from children in Gwangju, Korea: 2014-2018

OBJECTIVES

Non-typhoid Salmonella infection is a major agent of food-borne outbreaks as well as individual cases worldwide. However, few studies on drug-resistant Salmonella strains, especially those recovered from young children, are available. Therefore, we determined the prevalence and characteristics of cephalosporin-resistant Salmonella isolates in the south-west region of Korea over a five-year period.

METHODS

Non-duplicate Salmonella clinical isolates were recovered from diarrhoeagenic patient specimens at 12 hospitals in Gwangju, Korea between January 2014 and December 2018. Antimicrobial susceptibility testing and molecular features of cephalosporin-resistant isolates were determined.

RESULTS

A total of 652 Salmonella isolates were collected and 48 cefotaxime-resistant Salmonella isolates (7.4%), that belonged to nine Salmonella serovars, were identified. These were S. Enteritidis, S. Typhimurium, S. I 4,[5],12:i:-, S. Virchow, S. Agona, S. Bareilly, S. Infantis, S. Newport, and S. Schleissheim. The prevalence rate increased from 5.3% in 2014 to 10.3% in 2018. S. Virchow (44.4%) showed significantly high resistant rate compared to the other serovars. PGFE genotyping revealed high genetic homogeneities among each Salmonella serovars, suggesting clonal dissemination of cephalosporin-resistant strains.

CONCLUSIONS

Progressive increases in carriage rates and the possibility of community outbreaks by cephalosporin-resistant Salmonella in young children may pose tangible public health threats.

Clonal dissemination of Salmonella enterica serovar albany with concurrent resistance to ampicillin, chloramphenicol, streptomycin, sulfisoxazole, tetracycline, and nalidixic acid in broiler chicken in Korea

The aim of this study was to determine the prevalence, serovar distribution, antimicrobial resistance, and genotypic analyses of the dominating serovars of Salmonella in chickens from a national study in Korea. Between 2017 and 2018, a total of 550 chicken samples were collected from the top 12 integrated broiler chicken operations in Korea. Salmonella was isolated from 117 (32.5%) chicken feces and 19 (10.0%) retail chicken meat sources. Ten serovars were identified, and the most common Salmonella serovar was Salmonella ser. Albany (50 isolates, 36.8%), followed by S. Enteritidis (38 isolates, 27.9%), and S. Montevideo (23 isolates, 16.9%) isolated from 6, 10, and 6 operations, respectively. A total of 35 (25.7%) isolates were with the ACSSuTN (ampicillin, chloramphenicol, streptomycin, sulfisoxazole, tetracycline, and nalidixic acid) resistance pattern, with high prevalence of this resistance pattern in S. Albany (29 isolates, 58.0%). A total of 35 PFGE types were identified among Salmonella isolates of the serovars Albany, Enteritidis, Virchow, Montevideo, and Senftenberg, while 11 distinct types of PFGE patterns were found among S. Albany isolates, which showed an overall homology similarity of higher than 85%. Among these 35 PFGE types, 22 PFGE types corresponded to 32 isolates from samples limited to one operation, and the other 13 PFGE types corresponded to 72 isolates from samples widely distributed among different operations. These results highlighted rapid colony dissemination of multidrug-resistant S. Albany in chicken all over Korea after it first appeared in 2016; furthermore, the spread of Salmonella colonies between various integrated operations was common, and several operations played an important role in Salmonella carriage and transmission in Korea.

The Occurrence of Antimicrobial-Resistant Salmonella enterica in Hatcheries and Dissemination in an Integrated Broiler Chicken Operation in Korea

Positive identification rates of Salmonella enterica in hatcheries and upstream breeder farms were 16.4% (36/220) and 3.0% (6/200), respectively. Among the Salmonella serovars identified in the hatcheries, S. enterica ser. Albany (17/36, 47.2%) was the most prevalent, followed by the serovars S. enterica ser. Montevideo (11/36, 30.6%) and S. enterica ser. Senftenberg (5/36, 13.9%), which were also predominant. Thirty-six isolates showed resistance to at least one antimicrobial tested, of which 52.8% (n = 19) were multidrug resistant (MDR). Thirty-three isolates (enrofloxacin, MIC ≥ 0.25) showed point mutations in the gyrA and parC genes. One isolate, S. enterica ser. Virchow, carrying the blaCTX-M-15 gene from the breeder farm was ceftiofur resistant. Pulsed-field gel electrophoresis (PFGE) showed that 52.0% S. enterica ser. Montevideo and 29.6% S. enterica ser. Albany isolates sourced from the downstream of hatcheries along the broiler chicken supply chain carried the same PFGE types as those of the hatcheries. Thus, the hatcheries showed a high prevalence of Salmonella isolates with high antimicrobial resistance and no susceptible isolate. The AMR isolates from hatcheries originating from breeder farms could disseminate to the final retail market along the broiler chicken supply chain. The emergence of AMR Salmonella in hatcheries may be due to the horizontal spread of resistant isolates. Therefore, Salmonella control in hatcheries, particularly its horizontal transmission, is important.

Prevalence and potential risk of Salmonella enterica in migratory birds from South Korea

Salmonella is a type of zoonotic bacteria that represents an economic and public health concern worldwide. Difficulties in sample collection from migratory birds mean little is known about their importance as a reservoir of Salmonella. The present study evaluated the prevalence and potential risk of Salmonella enterica in migratory birds. From 2012-2017, 3661 cloacal swabs from migratory birds were collected in South Korea and tested to isolate S. enterica. Strains were tested for antimicrobial resistance and the presence of virulence genes. Thirty-six S. enterica strains, including S. enterica serovar Typhimurium (n = 19), S. Berta (n = 16), and S. Virchow (n = 1), were isolated from 34 birds. Two migratory birds were simultaneously co-infected with two serotypes. S. enterica was isolated from the Mallard duck, Northern pintail, Eurasian wigeon, Spot-billed duck, Eastern great egret, and Intermediate egret. S. Virchow was resistant to ciprofloxacin, with a point mutation (Ser-83-Phe) in the gyrA gene. Ten virulence genes were detected; sixteen strains were positive for all ten virulence genes. Salmonella was isolated from different migratory bird species and geographic locations with up to 100 % similarity of PFGE type. Eight S. Virchow strains taken from migratory birds, poultry farms, and chicken meat showed the same PFGE type. Salmonella was transmitted across species, space, and time in migratory birds. These birds may play a role in the dispersal of pathogenic and antimicrobial-resistant Salmonella and sporadic Salmonella infections in poultry; therefore, they may represent a direct or indirect public health threat.

Food Workers as a Reservoir of Extended-Spectrum-Cephalosporin-Resistant Salmonella Strains in Japan

Dissemination of extended-spectrum-cephalosporin (ESC)-resistant Salmonella, especially extended-spectrum-β-lactamase (ESBL)-producing Salmonella, is a concern worldwide. Here, we assessed Salmonella carriage by food workers in Japan to clarify the prevalence of ESC-resistant Salmonella harboring blaCTX-M We then characterized the genetic features, such as transposable elements, of blaCTX-M-harboring plasmids using whole-genome sequencing. A total of 145,220 stool samples were collected from food workers, including cooks and servers from several restaurants, as well as food factory workers, from January to October 2017. Isolated salmonellae were subjected to antimicrobial susceptibility testing (disk diffusion method), and whole-genome sequencing was performed for Salmonella strains harboring blaCTX-M Overall, 164 Salmonella isolates (0.113%) were recovered from 164 samples, from which we estimated that at least 0.113% (95% confidence interval [CI]: 0.096 to 0.132%) of food workers may carry Salmonella Based on this estimation, 3,473 (95% CI = 2,962 to 4,047) individuals among the 3,075,330 Japanese food workers are likely to carry Salmonella Of the 158 culturable isolates, seven showed resistance to ESCs: three isolates harbored blaCMY-2 and produced AmpC β-lactamase, while four ESBL-producing isolates harbored blaCTX-M-14 (n = 1, Salmonella enterica serovar Senftenberg) or blaCTX-M-15 (n = 3, S. enterica serovar Haardt). blaCTX-M-15 was chromosomally located in the S Haardt isolates, which also contained ISEcp1, while the S Senftenberg isolate contained an IncFIA(HI1)/IncHI1A/IncHI1B(R27) hybrid plasmid carrying blaCTX-M-14 along with ISEcp1 This study indicates that food workers may be a reservoir of ESBL-producing Salmonella and associated genes. Thus, these workers may contribute to the spread of blaCTX-M via plasmids or mobile genetic elements such as ISEcp1IMPORTANCE Antimicrobial-resistant Salmonella bacteria arise in farm environments through imprudent use of antimicrobials. Subsequently, these antimicrobial-resistant strains, such as extended-spectrum-β-lactamase (ESBL)-producing Salmonella, may be transmitted to humans via food animal-derived products. Here, we examined Salmonella carriage among food handlers in Japan. Overall, 164 of 145,220 fecal samples (0.113%) were positive for Salmonella Among the 158 tested isolates, four were identified as ESBL-producing isolates carrying ESBL determinants blaCTX-M-15 or blaCTX-M-14 In all cases, the genes coexisted with ISEcp1, regardless of whether they were located on the chromosome or on a plasmid. Our findings suggest that food workers may be a reservoir of ESBL-producing strains and could contribute to the spread of resistance genes from farm-derived Salmonella to other bacterial species present in the human gut.

Rapid Emergence of Florfenicol-Resistant Invasive Non-Typhoidal Salmonella in China: A Potential Threat to Public Health

Infection caused by invasive Salmonella occurs when Salmonella bacteria, which normally cause diarrhea, enter the bloodstream and spread through the body. We report the dramatic increase in florfenicol-resistant invasive non-typhoidal Salmonella (iNTS) in China between 2007 and 2016. Of the 186 iNTS strains isolated during the study period, 34 were florfenicol resistant, most of which harbored known resistance genes. Florfenicol is exclusively used in veterinary medicine in China, but now florfenicol-resistant iNTS is found in clinical patients. This finding indicates that antimicrobial resistance produced in veterinary medicine can be transmitted to humans, which poses a severe threat to public health.

Molecular characteristics of extended-spectrum β-lactamase/AmpC-producing Salmonella enterica serovar Virchow isolated from food-producing animals during 2010-2017 in South Korea

Global dissemination of non-typhoidal Salmonella producing extended-spectrum β-lactamase (ESBL) is a public-health concern. Recently, the prevalence of Salmonella spp. resistant to third-generation cephalosporins has been increasing in food-producing animals in Korea. In this study, we investigated resistance mechanisms and molecular characteristics of S. Virchow isolates resistant to extended-spectrum cephalosporins (ESCs). We obtained 265 S. Virchow isolates from fecal and carcasses samples of cattle (n = 2), pigs (n = 7), and chickens (n = 256) during 2010-2017, and observed high ESC-resistance (63.8%, 169/265); most of the resistant isolates (96.4%) were obtained from chickens. ESC-resistant S. Virchow isolates (n = 169) showed significantly higher resistance rates to other antimicrobials (especially aminoglycosides and tetracycline, p-value <0.0001), as well as prevalence of multidrug resistance, than did ESC-susceptible S. Virchow isolates (n = 96). All ESC-resistant S. Virchow produced CTX-M-15-type ESBL (n = 147) and/or CMY-2-type AmpC β-lactamase (n = 23). ESC-resistant S. Virchow represented seven pulsotypes, predominantly composed of type II (58.6%) and III (26.0%), detected in 69 farms in 10 provinces, and 33 farms in 7 provinces, respectively. Genes encoding ESC-resistance were horizontally transferred by conjugation to recipient E. coli J53; this was demonstrated in 28.8% (42/146) of bla_CTX-M-15-positive isolates and in 50.0% (11/22) of bla_CMY-2-positive isolates. All conjugative plasmids carrying bla_CTX-M-15 and bla_CMY-2 genes belonged to ST2-IncHI2 and ST12/CC12-IncI1, respectively. Genetic features of transferred bla genes were involved with ISEcp1 in both bla_CTX-M-15 and bla_CMY-2; ISEcp1 plays a critical role in the efficient capture, expression, and mobilization of bla genes. In addition to bla_CTX-M-15 genes, resistance markers to aminoglycosides and/or tetracycline were co-transferred to recipient E. coli J53. Our results show a high prevalence of ESBL-producing S. Virchow in chickens and chicken carcasses. Specific bla_CTX-M-15 and bla_CMY-2-carrying S. Virchow clones and plasmids were predominant in food-producing animals nationwide. Restriction of antimicrobial use and proper biosecurity practices at the farm level should be urgently implemented in the poultry industry.

Prevalence of Multidrug Resistant Salmonellae with Increasing Frequency of Salmonella enterica Serovars Kentucky and Virchow among Hospitalized Diarrheal Cases in and around Delhi, India

Non-typhoidal salmonellae (NTS) are a major cause of acute diarrhea with characteristic multidrug resistance (MDR). In a hospital-based study, 81 NTS were isolated and tested for serotypes and antimicrobial resistance (AMR). Salmonella enterica isolates were classified into 7 different typable serovars, however, 19 (23%) isolates remained untypable. The most common serovars were S. Kentucky (48%), and S. Virchow (22%). Most of the NTS isolates displayed resistance to nalidixic acid (NA) (73%), ciprofloxacin (CIP) (48%), ampicillin (AM) and norfloxacin (NOR) (36% each), and gentamicin (CN) (31%). The AMR profiles for CN and NA; and AM, CIP, NA and NOR, were found to be high in S. Virchow (83%) and S. Kentucky (43%), respectively. Analysis of the pulsed-field gel electrophoresis patterns of S. Kentucky revealed 3 clusters. S. Kentucky has clones closely related to become prominent in recent years in Delhi. The AMR appears to be consistent with the change in MDR patterns during 2014-2017. The observed prevalence of S. Kentucky and S. Virchow in large numbers of diarrheal cases is novel. The NTS are mostly resistant to fluoroquinolones, which is the current drug of choice for treating diarrheal cases. MDR is very common among clonally related S. Kentucky.

Characteristics of third-generation cephalosporin-resistant Salmonella from retail chicken meat produced by integrated broiler operations

Integrated broiler operations, which control and operate vertically through all phases of the chicken industry, have applied biosecurity and sanitation practices, housing technologies, feeding regimens, and antibiotic applications in different ways to improve food safety. The objective of this study was to compare the prevalence and antimicrobial resistance profiles of Salmonella isolates recovered from 6 different integrated broiler operations and to analyze the characteristics of extended-spectrum β-lactamase (ESBL)- and plasmid-mediated AmpC β-lactamase (pAmpC)-producing Salmonella isolates. Among 336 chicken meat samples, 57 were observed to be positive for Salmonella. However, the prevalence varied from 6.8% to 45.8% in chicken meat, indicating variations in Salmonella occurrence among the operations. Salmonella Albany was the dominant serovar, followed by Salmonella Virchow. In the antimicrobial resistance test, nalidixic acid-resistant isolates were the most prevalent (73.7%), followed by isolates resistant to ampicillin (49.1%) and tetracycline (42.1%). Among 14 third-generation cephalosporin-resistant isolates, 9 (64.3%) ESBL/pAmpC-producing isolates were only obtained from 2 operations: blaCTX-M-15 (n = 7) and blaCTX-M-79 (n = 1) for ESBL genes and blaCMY-2 (n = 1) for pAmpC. All ESBL/pAmpC-positive isolates exhibited high minimum inhibitory concentrations (≥128 μg/mL) of most cephalosporins and showed multidrug resistance. The transfer of ESBL/pAmpC genes was confirmed in transconjugants, which had the same genes and similar resistance patterns as those of the donor. Our findings suggest that Salmonella with resistance to third-generation cephalosporins can now be found in association with integrated broiler operations, providing data to support the development of monitoring and prevention programs for the development and spread of antimicrobial resistance in integrated broiler operations.

The Use of a Combined Bioinformatics Approach to Locate Antibiotic Resistance Genes on Plasmids From Whole Genome Sequences of Salmonella enterica Serovars From Humans in Ghana

In the current study, we identified plasmids carrying antimicrobial resistance genes in draft whole genome sequences of 16 selected Salmonella enterica isolates representing six different serovars from humans in Ghana. The plasmids and the location of resistance genes in the genomes were predicted using a combination of PlasmidFinder, ResFinder, plasmidSPAdes and BLAST genomic analysis tools. Subsequently, S1-PFGE was employed for analysis of plasmid profiles. Whole genome sequencing confirmed the presence of antimicrobial resistance genes in Salmonella isolates showing multidrug resistance phenotypically. ESBL, either blaTEM52-B or blaCTX-M15 were present in two cephalosporin resistant isolates of S. Virchow and S. Poona, respectively. The systematic genome analysis revealed the presence of different plasmids in different serovars, with or without insertion of antimicrobial resistance genes. In S. Enteritidis, resistance genes were carried predominantly on plasmids of IncN type, in S. Typhimurium on plasmids of IncFII(S)/IncFIB(S)/IncQ1 type. In S. Virchow and in S. Poona, resistance genes were detected on plasmids of IncX1 and TrfA/IncHI2/IncHI2A type, respectively. The latter two plasmids were described for the first time in these serovars. The combination of genomic analytical tools allowed nearly full mapping of the resistance plasmids in all Salmonella strains analyzed. The results suggest that the improved analytical approach used in the current study may be used to identify plasmids that are specifically associated with resistance phenotypes in whole genome sequences. Such knowledge would allow the development of rapid multidrug resistance tracking tools in Salmonella populations using WGS.

Bloodstream Infection Due to CTX-M-15 and TEM-1 Extended-Spectrum β-Lactamase-Producing Salmonella enterica serovar Virchow ST16

A 57-year-old man presented with high fever and diarrhea. A blood culture revealed the presence of a Group C nontyphoidal Salmonella (NTS) isolate. On Salmonella serotyping, the isolate was identified as Salmonella enterica serovar Virchow. Its sequence type was determined to be ST16 by sequence analysis of 7 different housekeeping genes. The bla_CTX-M group 1 and bla_TEM genes were amplified using multiplex PCR assay for detecting extended-spectrum β-lactamases (ESBL) genes. Sequences of both amplicons were respectively identical to CTX-M-15- and TEM-1-encoding genes. Since NTS is a cause of foodborne illness outbreaks in communities and an important cause of community-acquired bloodstream infection, clinicians should consider ESBL- or AmpC-producing NTS species in the differential diagnosis.

Nosocomial infection and its molecular mechanisms of antibiotic resistance

Nosocomial infection is a kind of infection, which is spread in various hospital environments, and leads to many serious diseases (e.g. pneumonia, urinary tract infection, gastroenteritis, and puerperal fever), and causes higher mortality than community-acquired infection. Bacteria are predominant among all the nosocomial infection-associated pathogens, thus a large number of antibiotics, such as aminoglycosides, penicillins, cephalosporins, and carbapenems, are adopted in clinical treatment. However, in recent years antibiotic resistance quickly spreads worldwide and causes a critical threat to public health. The predominant bacteria include Methicillin-resistant Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, and Acinetobacter baumannii. In these bacteria, resistance emerged from antibiotic resistant genes and many of those can be exchanged between bacteria. With technical advances, molecular mechanisms of resistance have been gradually unveiled. In this review, recent advances in knowledge about mechanisms by which (i) bacteria hydrolyze antibiotics (e.g. extended spectrum β-lactamases, (ii) AmpC β-lactamases, carbapenemases), (iii) avoid antibiotic targeting (e.g. mutated vanA and mecA genes), (iv) prevent antibiotic permeation (e.g. porin deficiency), or (v) excrete intracellular antibiotics (e.g. active efflux pump) are summarized.