Virulence region of plasmid pNL2001 of Salmonella enteritidis

Prevalence and antibiotic resistance of Salmonella in organic and non-organic chickens on the Eastern Shore of Maryland, USA

Introduction

Salmonella infections have been intensely increasing and becoming a universal public health crisis. This study investigated the prevalence of Salmonella in organic and non-organic chickens and the antimicrobial resistance profiles and virulence genes (invA, pagC, and spvC) in recovered Salmonella isolates.

Methods

Whole chicken carcasses [organic (n = 240) and non-organic (n = 240)] were obtained monthly for 1 year (n = 480) from a retail store on the Eastern Shore of Maryland. Salmonella isolation and identification were conducted by following the whole carcass enrichment method recommended by USDA-FSIS. Confirmed Salmonella isolates (organic n = 76; non-organic n = 137) were serotyped and tested for antibiotic susceptibility and virulence genes using standard methods.

Results

Forty-nine percent (237/480) of the carcasses were positive for Salmonella. Organic and non-organic positivity rates were 37.1 and 61.8%, respectively. A significantly higher Salmonella contamination was observed in non-organic chickens (p < 0.05). The most common serovars were Salmonella Kentucky (47%), S. Infantis (35%), S. Enteritidis (6%), S. Typhimurium (5%), and S. Blockley (4%). Isolates were frequently resistant to at least one antibiotic (91.24%) or multidrug resistant (45.54%). Resistance was observed to tetracycline (82.8%), minocycline (42.3%), nitrofurantoin (40.3%), cefazolin (38.3%), ampicillin (32.1%), and ceftriaxone (26%). All isolates were susceptible to fluoroquinolone, carbapenem, and glycylcycline. The majority of isolates (99.1%) possessed at least one of three virulence genes of concern and 4.2% tested positive for all three. Ninety-five, 89, and 6.6% of isolates contained invA, pagC, and spvC genes, respectively. The spvC gene was not detected in serovars recovered from organic chickens though 92% and 82% of isolates were positive for invA and pagC. The frequency of Salmonella recovered from non-organic chickens possessing invA, pagC, and spvC genes were 97.1, 89.8, and 10.2%, respectively. Detection of invA and pagC genes showed no significant difference (p > 0.05) between organic and non-organic chickens but a significantly higher spvC gene (p < 0.05) was detected in non-organic chickens due to the majority of S. Enteritidis (92.3%) exclusively recovered from non-organic chicken carried spvC gene.

Discussion

This study reveals a high prevalence of Salmonella in both organic and non-organic chickens, which exhibit resistance to vital antibiotics and carry virulence genes, thereby creating a potential risk of salmonellosis.

Molecular characterization of antibiotic resistant Salmonella Typhimurium and Salmonella Kentucky isolated from pre- and post-chill whole broilers carcasses

There is conflicting data regarding whether commercial chilling has any effect on persistence of Salmonella serovars, including antibiotic resistant variants, on chicken carcasses. A total of 309 Salmonella Typhimurium and Salmonella Kentucky isolates recovered from pre- and post-chill whole broiler carcasses were characterized for genetic relatedness using Pulsed Field Gel Electrophoresis (PFGE) and for the presence of virulence factors (invA, pagC, spvC) by PCR and for aerobactin and colicin production by bioassays. A subset of these isolates (n = 218) displaying resistance to either sulfisoxazole and/or ceftiofur [S. Typhimurium (n = 66) and S. Kentucky (n = 152)] were further tested for the presence of associated antibiotic resistance elements (class-I integrons and blaCMY genes) by PCR. All 145 ceftiofur resistant S. Kentucky and S. Typhimurium isolates possessed blaCMY genes. Class-I integrons were only detected in 6.1% (n = 4/66) of sulfisoxazole resistant S. Typhimurium isolates. The PFGE analysis revealed the presence of genetically diverse populations within the recovered isolates but clusters were generally concordant with serotypes and antimicrobial resistance profiles. At a 100% pattern similarity index, thirty-six percent of the undistinguishable S. Typhimurium and 22% of the undistinguishable S. Kentucky isolates were recovered from the same chilling step. All isolates possessed the invA and pagC genes, but only 1.4%possessed spvC. Irrespective of the chilling step, there was a significant difference (P < 0.05) in the production of aerobactin and colicin between S. Typhimurium and S. Kentucky isolates. Taken together, these results indicate that chilling impacted the recovery of particular Salmonella clonal groups but had no effect on the presence of class-I integrons, blaCMY genes, and tested virulence factors.

Molecular analysis of Salmonella isolates recovered from processed Turkey carcasses

This study was carried out to determine the prevalence of some virulence characteristics associated with Salmonella isolates recovered from processed turkey carcasses in the Midwestern region of the United States. A total of 94 Salmonella isolates recovered from turkey carcasses from two processing plants (A and B) were examined to determine the prevalence of invA, pagC, and spvC genes. Bioassays also were used to evaluate aerobactin and colicin production. All isolates (100%) were positive for the presence of invA and pagC but were negative for spvC. Overall, 19.1% of all isolates tested were positive for aerobactin production, and 25.5% of all isolates were positive for colicin. Aerobactin and colicin production differed among isolates recovered from the two plants; more isolates from plant B produced these compounds. The Salmonella isolates examined in this study possess significant potential for causing human illness.

Salmonella serotype Virchow causing salmonellosis in a Spanish region. Characterization and survey of clones by DNA fingerprinting, phage typing and antimicrobial resistance

The diversity of Salmonella serotype Virchow organisms causing human salmonellosis in a Spanish region over 1990-1996 was studied by genetic and phenotypic procedures. Isolates showing identical DNA fingerprintings (ribotypes, RAPD-, REP- and ERIC-types) were clustered into the same lineage. Eight lineages were defined, of which only one caused diseases throughout the studied period. Eleven phage types (PTs) were represented, the most frequent being PTs 8, 19, 31, throughout the study period, and PT4a only during 1994. Class I integrons with variable regions of 1000-, 1600-, and 2300-bp in size were respectively present in 24, 3 and 5 multiresistant isolates; 43.5% of isolates were susceptible to antimicrobials, the rest were grouped into 17 R-profiles, including from one up to eight resistances. Plasmids could be recovered from 71.5% of isolates and grouped into 25 plasmid profiles (with 1-7 plasmids each); a 3.6 kb cryptic-plasmid and a 60 kb virulence-plasmid were those most frequently found. Phage type, presence and size of integrons, and resistance profile were used to differentiate 39 clones. During the period studied 135 cases of Virchow salmonellosis were identified; 93 were apparently sporadic whereas the remainder were associated with four outbreaks. Infants under 1 year constituted the most frequent age group, with 30 gastroenteritis and two septicaemia episodes. In the four outbreaks, different clones falling into the prevalent lineage were implicated but each clone was involved in only one outbreak.

Natural pathogens of laboratory mice, rats, and rabbits and their effects on research

Laboratory mice, rats, and rabbits may harbor a variety of viral, bacterial, parasitic, and fungal agents. Frequently, these organisms cause no overt signs of disease. However, many of the natural pathogens of these laboratory animals may alter host physiology, rendering the host unsuitable for many experimental uses. While the number and prevalence of these pathogens have declined considerably, many still turn up in laboratory animals and represent unwanted variables in research. Investigators using mice, rats, and rabbits in biomedical experimentation should be aware of the profound effects that many of these agents can have on research.

Genetic map of the virulence plasmid of Salmonella enteritidis and nucleotide sequence of its replicons

Partial sequencing of a genomic library of the virulence plasmid of Salmonella enteritidis has been used to localize in the restriction map of this plasmid the genetic loci already described in other Salmonella plasmids. The comparison of the vestigial tra region with the corresponding genes in the F plasmid allowed us to define the extent of the deletions that the S. enteritidis plasmid should have suffered. The putative replicons of the plasmid, repB and repC, were isolated and both proved to be functional in Escherichia coli, but repC was segregationally unstable. The nucleotide sequence of repB showed the typical organization of RepFIIA replicons, although the similarity was lower than usual in this group of replicons. The highest homology was found with the replicon of the virulence plasmid pYVe439-80 from Yersinia enterocolitica (72.5%). Replicon repC also showed a maximum identity of 72.6% with known replicons, namely the RepFIB of pColV-K30 and P307, both virulence plasmids isolated from E. coli. We conclude that the S. enteritidis plasmid could arise from the S. typhimurium plasmid through deletions, and that they are evolutionary distant from other IncFI and IncFII plasmids.

Role of Escherichia coli rpoS and associated genes in defense against oxidative damage

The first phenotype described for mutations in the Escherichia coli rpoS gene was hypersensitivity to near-ultraviolet radiation and to its oxidative photoproduct, hydrogen peroxide. Initially named nur, this gene is now known to code for a sigma factor, and has acquired new names such as katF and rpoS. The role of its protein product (sigma-38) is to regulate a battery of genes as cells enter and rest in stationary phase. Some of the gene products are involved in protection against oxidants (e.g., catalases) and repair of oxidative damage (e.g., exonuclease III). Sigma-38 may also modulate transcription of certain growth phase genes, including hydroperoxidase I and glutathione reductase. Sigma-38 activity is regulated at transcriptional, translational, and protein stabilization levels. This review describes the complex mechanisms whereby sigma-38 controls various genes, the interaction of sigma-38 with other regulators, and a possible role of sigma-38 in bacterial virulence.