This study provides an estimate of antimicrobial resistance in intestinal indicator bacteria from humans (n = 97) and food animals (n = 388) in Tanzania. More than 70% of all fecal samples contained tetracycline (TE), sulfamethoxazole (STX), and ampicillin (AMP)-resistant coliforms, while cefotaxime (CTX)-resistant coliforms were observed in 40% of all samples. The average Log colony forming units/g of CTX-resistant coliforms in samples from humans were 2.20. Of 390 Escherichia coli tested, 66.4% were resistant to TE, 54.9% to STX, 54.9% to streptomycin, and 36.4% to CTX. Isolates were commonly (65.1%) multiresistant. All CTX-resistant isolates contained bla gene type. AMP- and vancomycin-resistant enterococci were rare, and the average concentrations in positive samples were low (log 0.9 and 0.4, respectively). A low-to-moderate resistance (2.1-15%) was detected in 240 enterococci isolates to the drugs tested, except for rifampicin resistance (75.2% of isolates). The average number of sulII gene copies varied between Log 5.37 and 5.68 with no significant difference between sample source, while cattle had significantly higher number of tetW genes than humans. These findings, based on randomly obtained samples, will be instrumental in designing antimicrobial resistance (AMR) intervention strategies for Tanzania.

This study did not indicate that transmission of antibiotic resistant bacteria is a frequent event following co-grazing of wild life and cattle.

Bacterial infections remain a threat to human and animal health worldwide, and there is an urgent need to find novel targets for intervention. In the current study we used a computer model of the metabolic network of Salmonella enterica serovar Typhimurium and identified pairs of reactions (cut sets) predicted to be required for growth in vivo. We termed such cut sets synthetic auxotrophic pairs. We tested whether these would reveal possible combined targets for new antibiotics by analyzing the performance of selected single and double mutants in systemic mouse infections. One hundred and two cut sets were identified. Sixty-three of these included only pathways encoded by fully annotated genes, and from this sub-set we selected five cut sets involved in amino acid or polyamine biosynthesis. One cut set (asnA/asnB) demonstrated redundancy in vitro and in vivo and showed that asparagine is essential for S. Typhimurium during infection. trpB/trpA as well as single mutants were attenuated for growth in vitro, while only the double mutant was a cut set in vivo, underlining previous observations that tryptophan is essential for successful outcome of infection. speB/speF,speC was not affected in vitro but was attenuated during infection showing that polyamines are essential for virulence apparently in a growth independent manner. The serA/glyA cut-set was found to be growth attenuated as predicted by the model. However, not only the double mutant, but also the glyA mutant, were found to be attenuated for virulence. This adds glycine production or conversion of glycine to THF to the list of essential reactions during infection. One pair (thrC/kbl) showed true redundancy in vitro but not in vivo demonstrating that threonine is available to the bacterium during infection. These data add to the existing knowledge of available nutrients in the intra-host environment, and have identified possible new targets for antibiotics.

Peroxiredoxins contribute to protection of some bacteria against reactive oxygen intermediates (ROIs) and reactive nitrogen intermediates (RNIs). Listeria monocytogenes, a facultative intracellular bacterial pathogen, interacts with ROIs and RNIs during infection. In this study, we investigated the involvement of the 2-Cys peroxiredoxin (Prx) homologue in L. monocytogenes in the protection against ROIs and RNIs and in virulence through the construction of an in-frame prx deletion mutant. The Δprx mutant had increased sensitivity to hydrogen peroxide and cumene hydroperoxide compared to the wild-type strain. The mutant also exhibited an increased susceptibility to the nitric oxide-generating compound S-nitroso-N-acetylpenicillamine (SNAP) and 3-morpholinosydnonimine hydrochloride (SIN-1), a peroxynitrite donor. Furthermore, a diminished virulence of the Δprx mutant relative to the wild-type was observed in C57BL/6 mice, but not in inducible nitric oxide synthase-deficient mice. The results suggest that Prx protects L. monocytogenes against oxidative and nitrosative stress in vitro and in vivo and that the prx-encoded polypeptide thereby is involved in L. monocytogenes virulence.

We investigated if the transcriptional response of Salmonella Typhimurium to temperature and acid variations was hysteretic, i.e. whether the transcriptional regulation caused by environmental stimuli showed memory and remained after the stimuli ceased. The transcriptional activity of non-replicating stationary phase cells of S. Typhimurium caused by the exposure to 45°C and to pH 5 for 30 min was monitored by microarray hybridizations at the end of the treatment period as well as immediately and 30 minutes after conditions were set back to their initial values, 25°C and pH 7. One hundred and two out of 120 up-regulated genes during the heat shock remained up-regulated 30 minutes after the temperature was set back to 25°C, while only 86 out of 293 down regulated genes remained down regulated 30 minutes after the heat shock ceased. Thus, the majority of the induced genes exhibited hysteresis, i.e., they remained up-regulated after the environmental stress ceased. At 25°C the transcriptional regulation of genes encoding for heat shock proteins was determined by the previous environment. Gene networks constructed with up-regulated genes were significantly more modular than those of down-regulated genes, implying that down-regulation was significantly less synchronized than up-regulation. The hysteretic transcriptional response to heat shock was accompanied by higher resistance to inactivation at 50°C as well as cross-resistance to inactivation at pH 3; however, growth rates and lag times at 43°C and at pH 4.5 were not affected. The exposure to pH 5 only caused up-regulation of 12 genes and this response was neither hysteretic nor accompanied of increased resistance to inactivation conditions. Cellular memory at the transcriptional level may represent a mechanism of adaptation to the environment and a deterministic source of variability in gene regulation.