Effect of Acid Adaptation on the Fate of Listeria monocytogenes in THP-1 Human Macrophages Activated by Gamma Interferon

Abstract

In Listeria monocytogenes the acid tolerance response (ATR) takes place through a programmed molecular response which ensures cell survival under unfavorable conditions. Much evidence links ATR with virulence, but the molecular determinants involved in the reactivity to low pHs and the behavior of acid-exposed bacteria within host cells are still poorly understood. We have investigated the effect of acid adaptation on the fate of L. monocytogenes in human macrophages. Expression of genes encoding determinants for cell invasion and intracellular survival was tested for acid-exposed bacteria, and invasive behavior in the human myelomonocytic cell line THP-1 activated with gamma interferon was assessed. Functional approaches demonstrated that preexposure to an acidic pH enhances the survival of L. monocytogenes in activated human macrophages and that this effect is associated with an altered pattern of expression of genes involved in acid resistance and cell invasion. Significantly decreased transcription of the plcA gene, encoding a phospholipase C involved in vacuolar escape and cell-to-cell spread, was observed in acid-adapted bacteria. This effect was due to a reduction in the quantity of the bicistronic plcA-prfA transcript, concomitant with an increase in the level(s) of the monocistronic prfA mRNA(s). The transcriptional shift from distal to proximal prfA promoters resulted in equal levels of the prfA transcript (and, as a consequence, of the inlA, hly, and actA transcripts) under neutral and acidic conditions. In contrast, the sodC and gad genes, encoding a cytoplasmic superoxide dismutase and the glutamate-based acid resistance system, respectively, were positively regulated at a low pH. Morphological approaches confirmed the increased intracellular survival and growth of acid-adapted L. monocytogenes cells both in vacuoles and in the cytoplasm of interferon gamma-activated THP-1 macrophages. Our data indicate that preexposure to a low pH has a positive impact on subsequent challenge of L. monocytogenes with macrophagic cells.

Surviving the Acid Test: Responses of Gram-Positive Bacteria to Low pH

“…Transcription of plcA, which is involved in vacuolar escape and cell-to-cell spread, is decreased by 12% after acid adaptation. In contrast, increased transcription of sodC (310%) and the combined gad genes (3000%) was observed after acid adaptation (47). It is interesting that while Hly and PlcA are both expressed in the phagosome and contribute to vacuolar escape, the pH ranges for their enzymatic activities are different (96,99).…”

“…Following activation of the macrophages, the rate of invasion of acid-adapted and nonadapted cells remained equal. However, the active macrophages became nonpermissive to growth of the nonadapted cells while acid-adapted cells demonstrated a greatly enhanced intracellular growth rate (47).…”

“…On the basis of these observations, it is speculated that after cell invasion, the acidic environment of the phagosome could prevent plcA expression while enabling Hly to be accumulated in an enzymatically active state. Pore formation by Hly would then raise the phagosomal pH to neutral values, thereby increasing PlcA expression (47). Such tight regulation is essential, since many virulence factors, including listerial hemolysin and phosphatidylinositolspecific phospholipase C (208), are highly antigenic for the host immune response.…”

Contributions of Listeria monocytogenes σ B and PrfA to expression of virulence and stress response genes during extra- and intracellular growth

“…While s B -dependent opuCA transcription under salt stress has been reported previously, the current study establishes reference values for transcript levels of s B -dependent genes in extracellular L. monocytogenes at 37 u C, which were needed to characterize relative s B activity in intracellular L. monocytogenes. As Milohanic et al (2003) showed that the s B -dependent opuCA also may be co-regulated by PrfA, we measured expression of a second s B -dependent gene, gadA, [lmo2434 (Conte et al, 2002); which is also designated gadD (Wemekamp-Kamphuis et al, 2004) and gadD3 (Cotter et al, 2005)]. s B activity (as determined by gadA and opuCA transcript levels) in intracellular L. monocytogenes arrested in either the host cell vacuole or the host cell cytosol was higher than in unstressed, exponentially growing bacteria and lower than in bacteria exposed to extracellular environmental stresses.…”

Transcriptome Analysis of Alkali Shock and Alkali Adaptation inListeria monocytogenes10403S

“…In what might appear to be contrary to the above processes of (producing=excreting) acid products, this study observed different levels of induction of decarboxylases such as the lysine decarboxylase (lmo2694) and glutamate decarboxylase (GAD) (lmo2434), which produce alkaline end products (Cotter et al, 2005) in AS and AA (Table 2). Induction of the latter (GAD) system is suggested to increase the pH of the immediate cell environment by a number of processes, including the export of g-aminobutyrate (Conte et al, 2002;Wemekamp-Kamphuis et al, 2004). The GAD system has been extensively reported as a key system in the adaptation of Listeria and other pathogens (Olier et al, 2004;Wemekamp-Kamphuis et al, 2004) to acid challenge in the human stomach (Blankenhorn et al, 1999).…”