The goal of this study was to follow ceftiofur-treated and untreated cattle in a normally functioning dairy to examine enteric Escherichia coli for changes in antibiotic resistance profiles and genetic diversity. Prior to treatment, all of the bacteria cultured from the cows were susceptible to ceftiofur. Ceftiofur-resistant E. coli was only isolated from treated cows during and immediately following the cessation of treatment, and the 12 bla CMY-2 -positive isolates clustered into two genetic groups. E. coli bacterial counts dropped significantly in the treated animals (P < 0.027), reflecting a disappearance of the antibiotic-susceptible strains. The resistant bacterial population, however, did not increase in quantity within the treated cows; levels stayed low and were overtaken by a returning susceptible population. There was no difference in the genetic diversities of the E. coli between the treated and untreated cows prior to ceftiofur administration or after the susceptible population of E. coli returned in the treated cows. A cluster analysis of antibiotic susceptibility profiles resulted in six clusters, two of which were multidrug resistant and were comprised solely of isolates from the treated cows immediately following treatment. The antibiotic treatment provided a window to detect the presence of ceftiofur-resistant E. coli but did not appear to cause its emergence or result in its amplification. The finding of resistant isolates following antibiotic treatment is not sufficient to estimate the strength of selection pressure nor is it sufficient to demonstrate a causal link between antibiotic use and the emergence or amplification of resistance.

Salmonella enterica serovar Typhimurium strain 798 is a clinical isolate from a pig and is known to be able to cause persistent, asymptomatic infections. This strain also is known to exist in two phenotypes (adhesive and nonadhesive to enterocytes) and can switch between the two phenotypes at a rate consistent with phase variation. Cells in the adhesive phenotype are more readily phagocytosed by leukocytes than nonadhesive cells. Once in a leukocyte, adhesive-phase cells survive while nonadhesive-phase cells die. In the present study, nonadhesive mutants were obtained with the transposon TnphoA. A nonadhesive mutant was selected for study and was shown by electron microscopy not to produce fimbriae. The gene encoding the adhesin was cloned and sequenced. Based on its sequence, the adhesin was shown to be FimA, the major subunit of type 1 fimbriae. The nonadhesive mutant was attenuated in its ability to colonize both mouse and pig intestines, but remained capable of systemic spread in mice. The nonadhesive mutant was phagocytosed to the same extent as parental cells in the adhesive phase and then survived intracellularly. These results demonstrated that type 1 fimbriae were important for attachment to enterocytes and promoted intestinal colonization. However, they were not important in promoting phagocytosis or intracellular survival.

This study evaluated the relationship between florfenicol resistance and flo genotypes in 1,987 Escherichia coli isolates from cattle. The flo gene was detected in 164 isolates, all of which expressed resistance to florfenicol at MICs of >256 g/ml. The florfenicol MICs for all isolates that lacked flo were <16 g/ml.Florfenicol is a fluorinated analog of chloramphenicol approved by the Food and Drug Administration in 1996 for the treatment of bovine respiratory disease (BRD) pathogens. The main florfenicol targets in respiratory diseases of cattle are Pasteurella multocida, Mannheimia haemolytica, and Haemophilus somnus. Florfenicol is occasionally used extra-label in the treatment of calf diarrhea (10) and can be effective against Escherichia coli strains isolated from diarrheic calves (9). Florfenicol binds to the 50S subunit of the bacterial ribosome and disrupts protein synthesis. E. coli strains have been recovered with a gene known as flo that expresses resistance to florfenicol and chloramphenicol and is typically located on large transferable plasmids (7). Neither the enzyme chloramphenicol acetyltransferase nor the nonenzymatic chloramphenicol resistance gene cmlA confers resistance to florfenicol (10).Past studies documented florfenicol resistance in animal isolates of E. coli. White et al. (10) found that for 44 of 48 neonatal calf diarrhea E. coli isolates, there was decreased susceptibility to florfenicol, flo was present, and the florfenicol MIC range was 16 to Ն256 g/ml. For 4 of the 48 E. coli isolates, cmlA was present, flo was not present, and the florfenicol MIC range was Յ8 to 64 g/ml. Keyes et al. (6) analyzed chloramphenicol-resistant E. coli from sick chickens and found that some isolates possessed the flo gene. The florfenicol MICs for isolates harboring flo were Ն32 g/ml, whereas the florfenicol MICs for isolates without flo were Յ8 g/ml. Bischoff et al. (1) analyzed 48 chloramphenicol-resistant E. coli isolates from cases of neonatal swine diarrhea. The florfenicol MIC for one isolate, which possessed flo, was 256 g/ml. The florfenicol MICs for the remaining 47 isolates ranged from 8 to Ն16 g/ml.Currently, there are no National Committee for Clinical Laboratory Standards (NCCLS) MIC breakpoints approved to indicate florfenicol resistance in E. coli, and many studies (1, 10) use MIC breakpoints for the BRD pathogens. When these guidelines are used, an MIC of Ն8 g/ml indicates resistance, an MIC of 4 g/ml indicates intermediate susceptibility, and an MIC of Յ2 g/ml indicates susceptibility. Previous studies demonstrated that the presence of flo is associated with very high florfenicol MICs (1, 6, 10). Therefore, we hypothesized that MIC breakpoints for BRD pathogens would not be useful in correlating the presence of flo in E. coli isolates with the observed florfenicol MIC. The objective of this study was to evaluate the relationship between florfenicol resistance phenotypes and flo genotypes in commensal bovine isolates of E. coli.During an intensive, longitudinal study of antimicrobial ...

Salmonella enterica is an important food borne pathogen that is frequently carried by swine. Carrier animals pose a food safety risk because they can transmit S. enterica to finished food products in the processing plant or by contamination of the environment. Environmental contamination has become increasingly important as non-animal foods (plant-based) have been implicated as sources of S. enterica. The prevalence of S. enterica in swine is high and yet carrier animals remain healthy. S. enterica has developed a highly sophisticated set of virulence factors that allow it to adapt to host environments and to cause disease. It is assumed that S. enterica also has developed unique ways to maintain itself in animals and yet not cause disease. Here we describe our research to understand persistence. Specifically, data are presented that demonstrates that detection of most carrier animals requires specific stresses that cause S. enterica to be shed from pigs. As well, we describe a phenotypic phase variation process that appears to be linked to the carrier state and a complex set of factors that control phenotypic phase variation. Finally, we describe how the composition of the gut bacterial microbiome may contribute to persistence and at the least how S. enterica might alter the composition of the gut bacterial microbiome.

Abstract. The objective of the present investigation was to differentiate between strains of Streptococcus equi subspecies equi implicated in abscess formation in vaccinated horses. Streptococcus equi isolates recovered from clinical specimens associated with equine strangles cases submitted to the University of Illinois Veterinary Diagnostic Laboratory were compared with S. equi isolates representing at least 12 lots of a commercial modified live vaccine (MLV) to determine whether the isolates obtained from the abscesses were vaccine or wild type. Genotyping techniques evaluated included enterobacterial repetitive intergenic consensus polymerase chain reaction (PCR), repetitive extragenic palindrome PCR, BOX element PCR, ribotyping, and pulsed-field gel electrophoresis (PFGE). Phenotypic evaluations were performed using the Biolog GP2 Microplate (hereafter, Biolog). In cases where Biolog and PFGE results did not coincide, a single nucleotide polymorphism located in the upstream regulatory region of szp gene was used to identify the S. equi strains. PFGE and Biolog successfully differentiated wild-type S. equi strains isolated from clinical submissions from isolates of the MLV. PFGE genotyping enabled further subtyping of the wild-type strains, whereas Biolog combined with szp sequencing was useful in differentiating the MLV strain from its wild-type progenitor. Deletion of a single guanine residue located in the upstream regulatory region of the szp gene appears to be conserved among vaccine isolates, and shows a 98.5% correlation to Biolog identification. This multiphasic approach can be used to answer specific diagnostic questions pertaining to the source of infection and/or outbreak, or to address quarantine concerns.