Adaptive acid tolerance response in Listeria monocytogenes: isolation of an acid-tolerant mutant which demonstrates increased virulence

Abstract

The ability of Listeria monocytogenes to tolerate low-pH environments is of particular importance because the pathogen encounters such environments in vivo, both during passage through the stomach and within the macrophage phagosome. In our study, L. monocytogenes was shown to exhibit a significant adaptive acid tolerance response following a 1-h exposure to mild acid (pH 5.5), which is capable of protecting cells from severe acid stress (pH 3.5). Susceptibility to pH 3.5 acid is growth phase dependent. Stationary-phase Listeria cultures are naturally resistant to the challenge pH (pH 3.5), while exponential-phase cultures require adaptation at pH 5.5 to induce acid tolerance. Adaptation requires protein synthesis, since treatment with chloramphenicol prevents the development of acid tolerance. Induction of the acid tolerance response also protects L. monocytogenes against the effects of other environmental stresses. Acid-adapted cells demonstrate increased tolerance toward thermal stress, osmotic stress, crystal violet, and ethanol. Following prolonged exposure of L. monocytogenes to pH 3.5, we isolated mutants which constitutively demonstrate increased acid tolerance at all stages of the growth cycle. These mutants do not display full acid tolerance, but their resistance to low pH can be further increased following adaptation to mild-acid conditions. The mutants demonstrated increased lethality for mice relative to that of the wild type when inoculated by the intraperitoneal route. When administered as lower inocula, the mutants reached higher levels in the spleens of infected mice than did the wild type. The data suggest that low-pH conditions may have the potential to select for L. monocytogenes mutants with increased natural acid tolerance and increased virulence.

Cell density dependent acid sensitivity in stationary phase cultures of enterohemorrhagic Escherichia coli O157:H7

“…Results from several laboratories have shown that the acid resistance in E. coli O157:H7 is much higher in the stationary phase than in the exponential phase [8,23]. Similar observations have also been made in Shigella [9], Salmonella [24] and Listeria [25]. Experiments described in previous sections established that stationary phase cells at high concentrations were more acid sensitive than the diluted cells and the increased acid sensitivity was also observed in the high concentrations of the stationary phase culture supernatant.…”

Effect of acid-adaptation on Listeria monocytogenes survival and translocation in a murine intragastric infection model

“…1). L. monocytogenes is able to mount an acid tolerance response [3^5, 9]. Therefore we tested whether prior adaptation to a mild acidic pH could enable L. monocytogenes to survive the intragastric transit.…”

“…Studies on the e¡ect of acid-adaptation on L. monocytogenes have shown that acid-tolerant mutants were of increased virulence for mice inoculated by the peritoneal route [9]; conversely, acid-sensitive mutants displayed reduction in virulence [10]. However, when wild-type L. monocytogenes were acid-adapted, the observed increase in acid tolerance did not lead to an increased virulence in this parenteral mouse model of infection [5].…”

Comparison of threeListeria monocytogenesstrains in a guinea-pig model simulating food-borne exposure

“…Examples of animal models carrying a homologue to human E-cadherin include a nonhuman primate model (Smith et al, 2003(Smith et al, , 2008 and a transgenic mouse model (Lecuit et al, 2001). Some models have bypassed the intestines by injecting the bacteria into the peritoneum (O'Driscoll et al, 1996;Takeuchi et al, 2006); others have used the intragastric route (Schlech et al, 1993;Faith et al, 2006) and/or have increased the survival of bacteria in the gastric environment, for example, by administration of sodium bicarbonate or calcium carbonate (Nightingale et al, 2008). Most of these models have thus not simulated the complete (unfacilitated) route of Listeria from food to faeces, or have not been applicable for differentiation between the virulence of investigated strains (Takeuchi et al, 2006).…”