How does Listeria monocytogenes combat acid conditions?
ABSTRACT Listeria monocytogenes, a major foodborne pathogen, possesses a number of mechanisms that enable it to combat the challenges posed by acidic environments, such as that of acidic foods and the gastrointestinal tract. One mechanism employed by L. monocytogenes for survival at low pH is the adaptive acid tolerance response (ATR) in which a short adaptive period at a nonlethal pH induces metabolic changes that allow the organism to survive a lethal pH. Overcoming acid conditions by L. monocytogenes involves a variety of regulatory responses, including the LisRK 2-component regulatory system, the SOS response, components of the σ(B) regulon, changes in membrane fluidity, the F0F1-ATPase proton pump, and at least 2 enzymatic systems that regulate internal hydrogen ion concentration (glutamate decarboxylase and arginine deiminase). It is not clear if these mechanisms exert their protective effects separately or in concert, but it is probable that these mechanisms overlap. Studies using mutants indicate that the glutamate decarboxylase system can protect L. monocytogenes when the organism is present in acidic juices, yogurt, salad dressing, mayonnaise, and modified CO2 atmospheres. The glutamate decarboxylase system also has a role in protecting L. monocytogenes against the acidic environment of the stomach. There is a need to study other acid resistance mechanisms of L. monocytogenes to determine their effectiveness in protecting the organism in acidic foods or during transit through the acid stomach.
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ABSTRACT: Despite the world-wide reports of outbreaks of food-borne listeriosis, the occurrence of Listeria is still not widely reported in Nigeria. This is possibly due to lack of a large cold storage food chain and the absence of a comprehensive surveillance system for food-borne pathogens. Searches carried out on major databases revealed that Listeria has been reported in humans, animals, environment and food in Nigeria. In Nigeria, the organism has been reported in pregnant women and neonates while ruminants dominate reports of occurrence in animals. In food especially fish, L. monocytogenes is reported more than any other Listeria species. The organism has been isolated from water bodies and soils from different environments in Nigeria. However, all reports on the occurrence of Listeria spp. were based on classical serotyping, biochemical tests and dark colouration of media due to hydrolysis of aesculin with no emerging pattern of infection or dominant molecular serotype. There is an opportunity to utilize the current polymerase chain reaction based molecular techniques to characterize Listeria spp. so that accurate information on existing Listeria strains and sources of infection can be established in all regions in Nigeria.International Food Research Journal 04/2015; 22(2):455.
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ABSTRACT: A common stress encountered by both pathogenic and environmental bacteria is exposure to a low-pH environment, which can inhibit cell growth and lead to cell death. One major defense mechanism against this stress is the arginine deiminase (ADI) pathway, which catabolizes arginine to generate two ammonia molecules and one molecule of ATP. While this pathway typically relies on the utilization of arginine, citrulline has also been shown to enter into the pathway and contribute to protection against acid stress. In the pathogenic bacterium Streptococcus pyogenes, the utilization of citrulline has been demonstrated to contribute to pathogenesis in a murine model of soft tissue infection, although the mechanism underlying its role in infection is unknown. To gain insight into this question, we analyzed a panel of mutants defective in different steps in the ADI pathway to dissect how arginine and citrulline protect S. pyogenes in a low-pH environment. While protection provided by arginine utilization occurred through the buffering of the extracellular environment, citrulline catabolism protection was pH-independent, requiring the generation of ATP via the ADI pathway and a functional F1F0-ATP synthase. This work demonstrates that arginine and citrulline catabolism protect against acid stress through distinct mechanisms, and have unique contributions to virulence during an infection. An important aspect of bacterial pathogenesis is the utilization of host-derived nutrients during an infection for growth and virulence. Previously published work from our lab identified a unique role for citrulline catabolism in Streptococcus pyogenes during a soft tissue infection. The current manuscript probes the role of citrulline utilization during this infection, and its contribution to protection against acid stress. This work reveals a unique and concerted action between the catabolism of citrulline and the F1F0-ATPase that function together to provide protection for bacteria in a low-pH environment. Dissection of these collaborative pathways highlights the complexity of bacterial infections, and the contribution of atypical nutrients, such as citrulline, to pathogenesis. Copyright © 2015, American Society for Microbiology. All Rights Reserved.Journal of Bacteriology 02/2015; 197(7). DOI:10.1128/JB.02517-14 · 2.69 Impact Factor
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ABSTRACT: Listeria monocytogenes, a food-borne pathogen, has the capacity to maintain intracellular pH (pHi) homeostasis in acidic environments, but the underlying mechanisms remain elusive. Here, we report a simple microplate-based fluorescent method to determine pHi of listerial cells that were prelabeled with the fluorescent dye carboxyfluorescein diacetate N-succinimidyl ester and subjected to acid stress. We found that L. monocytogenes responds differently among strains toward organic and inorganic acids to maintain pHi homeostasis. The capacity of L. monocytogenes to maintain pHi at extracellular pH 4.5 (pHex) was compromised in the presence of acetic acid and lactic acid, but not by hydrochloric acid and citric acid. Organic acids exhibited more inhibitory effects than hydrochloric acid at certain pH conditions. Furthermore, the virulent stains L. monocytogenes EGDe, 850658 and 10403S was more resistant to acidic stress than the avirulent M7 which showed a defect in maintaining pHi homeostasis. Deletion of sigB, a stress-responsive alternative sigma factor from 10403S, markedly altered intracellular pHi homeostasis, and showed a significant growth and survival defect under acidic conditions. Thus, this work provides new insights into bacterial survival mechanism to acidic stresses.Frontiers in Microbiology 01/2015; 6:15. DOI:10.3389/fmicb.2015.00015 · 3.94 Impact Factor