Abstract
BackgroundMultidrug-resistant (MDR) Salmonella enterica serovar Typhimurium (S. Typhimurium) is a serious public health threat as infections caused by these strains are more difficult and expensive to treat. Livestock serve as a reservoir for MDR Salmonella, and the antibiotics chlortetracycline and florfenicol are frequently administrated to food-producing animals to treat and prevent various diseases. Therefore, we evaluated the response of MDR S. Typhimurium after exposure to these two antibiotics.ResultsWe exposed four MDR S. Typhimurium isolates to sub-inhibitory concentrations of chlortetracycline (16 and 32 µg/ml) or florfenicol (16 µg/ml) for 30 min during early-log phase. Differentially expressed genes following antibiotic treatment were identified using RNA-seq, and genes associated with attachment and those located within the Salmonella pathogenicity islands were significantly up-regulated following exposure to either antibiotic. The effect of antibiotic exposure on cellular invasion and motility was also assessed. Swimming and swarming motility were decreased due to antibiotic exposure. However, we observed chlortetracycline enhanced cellular invasion in two strains and florfenicol enhanced invasion in a third isolate.ConclusionsChlortetracycline and florfenicol exposure during early-log growth altered the expression of nearly half of the genes in the S. Typhimurium genome, including a large number of genes associated with virulence and pathogenesis; this transcriptional alteration was not due to the SOS response. The results suggest that exposure to either of these two antibiotics may lead to the expression of virulence genes that are typically only transcribed in vivo, as well as only during late-log or stationary phase in vitro.
Highlights
IntroductionLivestock serve as a reservoir for MDR Salmonella, and the antibiotics chlortetracycline and florfenicol are frequently administrated to food-producing animals to treat and prevent various diseases
We found that exposure to sub-inhibitory concentrations of either antibiotic led to over 50% of the genes in the genome being differentially regulated, resulting in changes in invasion and motility phenotypes
In liquid culture during early-log growth, Salmonella display little-to-no invasiveness as motility expression is up-regulated while Salmonella pathogenicity islands (SPI) and attachment gene expression are down-regulated
Summary
Livestock serve as a reservoir for MDR Salmonella, and the antibiotics chlortetracycline and florfenicol are frequently administrated to food-producing animals to treat and prevent various diseases. S. Typhimurium strains following exposure to subinhibitory levels of tetracycline and chloramphenicol for 30 min during the non-invasive early-log growth phase [5]. Resistance to tetracycline and chloramphenicol is mediated by the specific efflux pumps tetA/G and floR in these MDR S. One particular MDR isolate that encoded tetBCD and the chloramphenicol-inactivation enzyme, cat, did not have significantly different gene expression profiles or invasion phenotypes due to either antibiotic. Exposure to ampicillin and streptomycin with enzymemediated resistance mechanisms did not affect invasion phenotypes nor did the antibiotics change the expression of the invasion pathway regulating the hilA gene (as observed with tetracycline and chloramphenicol). It is possible that efflux-encoded resistance mechanisms are required for the considerable transcriptomic and phenotypic differences that were observed
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