Listeria Monocytogenes: A Pathological Perspective: Difference between revisions

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&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Specific antibiotic resistance for streptomycin, erythromycin, tetracycline, and chloramphenicol in <i>L. monocytogenes</i> has traced to plasmid pIP811, which is easily transferable among common gram-positive bacteria.  The genes in this plasmid were extremely similar to those with the same resistance capacities found in <i>Enterococci</i> or <i>Streptococci</i>; one gene (responsible for chloramphenicol resistance) is even homologous to its counterpart in <i>Enterococci</i> and <i>Streptococci</i> (23). <br>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Specific antibiotic resistance for streptomycin, erythromycin, tetracycline, and chloramphenicol in <i>L. monocytogenes</i> has traced to plasmid pIP811, which is easily transferable among common gram-positive bacteria.  The genes in this plasmid were extremely similar to those with the same resistance capacities found in <i>Enterococci</i> or <i>Streptococci</i>; one gene (responsible for chloramphenicol resistance) is even homologous to its counterpart in <i>Enterococci</i> and <i>Streptococci</i> (23). <br>
[[File: cephalosporins.png|thumb|right|300px| Figure 5: General skeleton structure of cephalosporin antibiotics.  http://goldbook.iupac.org/C00939.html]]
[[File: cephalosporins.png|thumb|right|300px| Figure 5: General skeleton structure of cephalosporin antibiotics.  http://goldbook.iupac.org/C00939.html]]
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; <i>L. monocytogenes</i> expresses uniquely high resistance to cephalosporins, which are typically used to treat gram-positive infections of unknown origin due to their strength.  The resistance is largely contributed to the gene liaS, which simultaneously increases the susceptibility of the cell to nisin.  Conversely, mutant lmo2229 increases cell resistance to nisin while decreasing resistance to cephalosporins.  In order to make <i>L. monocytogenes</i> as susceptible to nisin as possible, research is constantly being done on the virulence factors of <i>L. monocytogenes</i> to find weaknesses to be exploited.  Recently, the TelA gene has been discovered to be extremely susceptible to both nisin and antibacterial medications.  The eventual goal is to implicate the susceptible sequences into <i>L. monocytogenes</i>, lowering the chances of it to cause foodborne illness (24).<br>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; <i>L. monocytogenes</i> expresses uniquely high resistance to cephalosporins, which are typically used to treat gram-positive infections of unknown origin due to their strength.  The resistance is largely contributed to the gene liaS, which simultaneously increases the susceptibility of the cell to nisin.  Conversely, mutant lmo2229 increases cell resistance to nisin while decreasing resistance to cephalosporins (Figure 5).  In order to make <i>L. monocytogenes</i> as susceptible to nisin as possible, research is constantly being done on the virulence factors of <i>L. monocytogenes</i> to find weaknesses to be exploited.  Recently, the TelA gene has been discovered to be extremely susceptible to both nisin and antibacterial medications.  The eventual goal is to implicate the susceptible sequences into <i>L. monocytogenes</i>, lowering the chances of it to cause foodborne illness (24).<br>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Another important gene discovered in <i>L. monocytogenes</i> that provides the bacteria with significant antibiotic resistance is CesRK.  This gene is responsible for the formation of many types of membrane proteins and has a two-part system.  Its resistance lies not only in ß-lactam antibiotics but also in its increased resistance to ethanol, which poses the possibility of CesRK as serving as a regulator for alcohol-induced genes within <i>L. monocytogenes</i>.  Eventually, this may become a problem, as many disinfectants are alcohols, meaning they will need to accommodate for the weakened effectiveness against <i>L. monocytogenes</i>, unfortunately one of the more dangerous species of bacteria (25).<br>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Another important gene discovered in <i>L. monocytogenes</i> that provides the bacteria with significant antibiotic resistance is CesRK.  This gene is responsible for the formation of many types of membrane proteins and has a two-part system.  Its resistance lies not only in ß-lactam antibiotics but also in its increased resistance to ethanol, which poses the possibility of CesRK as serving as a regulator for alcohol-induced genes within <i>L. monocytogenes</i>.  Eventually, this may become a problem, as many disinfectants are alcohols, meaning they will need to accommodate for the weakened effectiveness against <i>L. monocytogenes</i>, unfortunately one of the more dangerous species of bacteria (25).<br>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Similar to the two-part functioning of CesRK, LisRK also is divided into a two-part system.  This time, however, the gene is in charge of osmoregulation of the cell, adapting easily to high levels of osmolarity in the conditions of either its host cell or in the outside environment.  As osmolarity is usually caused by a change in salt levels, HtrA assists LisRK greatly when trying to adjust osmolarity with the filament formation (26).  Additionally, LisRK can adapt to pH stresses and hydrogen peroxide stresses elegantly without facing too much of a burden from the adjustment (27).
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Similar to the two-part functioning of CesRK, LisRK also is divided into a two-part system.  This time, however, the gene is in charge of osmoregulation of the cell, adapting easily to high levels of osmolarity in the conditions of either its host cell or in the outside environment.  As osmolarity is usually caused by a change in salt levels, HtrA assists LisRK greatly when trying to adjust osmolarity with the filament formation (26).  Additionally, LisRK can adapt to pH stresses and hydrogen peroxide stresses elegantly without facing too much of a burden from the adjustment (27).

Revision as of 22:09, 7 May 2014

Edited By: Dominic Camperchioli

Physiology

Figure 1: Scanning Electron Micrograph of Listeria monocytogenes, colorized. The actin filaments can be easily seen extending from the cell body. http://www.safefood.eu/SafeFood/media/SafeFoodLibrary/Images/Consumer/Preparing%20Food/listeria4-(2).jpg


          Listeria monocytogenes is an aerobic gram-positive bacillus commonly found in pregnant women, newborns, and people with compromised immune systems, and is motile via flagella (Figure 1). As L. monocytogenes is a common bacterium in the soil, it often is consumed by animals and from there become a food borne illness for humans. Usually, listeriosis is a result of dairy consumption (1), but it can be transmitted through other sorts of foods, such as the Listeria outbreak that originated at a Colorado cantaloupe farm (2).

          However, listeriosis is not as common as one might imagine, as most healthy individuals are not normally affected by small amounts of L. monocytogenes (1). Even though it is commonly associated with meningitis, most infections begin in the gastrointestinal tract, specifically the small intestine, where macrophages are common. From there, the infection will generally spread to surrounding tissues, such as epithelial cells and hepatocytes, before migrating up to the brain (3). Although rare, listeriosis is lethal in about 30% of cases (4).

          The bacteria prosper intracellularly, preferring to be inside of macrophages, where conditions are optimal for growth and replication; the bacteria actually survive much worse extracellularly, as the means for growth and replication are not as easily accessible as when they are in a host cell. Although antibodies and vaccinations are not effective against L. monocytogenes, acquired immunity is possible, although tricky (5).

          This page starts with a general overview of the life cycle of L. monocytogenes and its virulence factors and then analyzes effects of antibiotics and host cell defense methods to summarize the status quo of Listeria monocytogenes from a pathological perspective.

Overview of L. monocytogenes

          Scientists have been aware of Listeria monocytogenes since the 1920’s (4), yet not everything about it is known. However, its life cycle has been studied carefully, as it is different from most pathogens. These differences lead to variations among effective treatment methods when infected with L. monocytogenes when compared to other common gram-positive pathogens, and its specific pathogenesis is unlike most known. As L. monocytogenes is infamous for its lethality of infection, it has and continues to be a hot topic for research (3). Therefore, to be able to effectively analyze the mechanisms used by hosts to ward off infection, one must first understand the life cycle and virulence factors of L. monocytogenes.

Life Cycle

This enitre section is summarized by the diagram in Figure 3.

Entry into Host Cells

Figure 2: Zipper mechanism of phagocytosis, a standard method used by many types of cells and in many organisms, including human macrophages. Figure from Tollis et al. 2010. http://www.biomedcentral.com/1752-0509/4/149/figure/F1

          Infection usually begins in the small intestine, where L. monocytogenes are engulfed easily by macrophages. However, L. monocytogenes can induce their own phagocytosis into nonphagocytic cells. This induction has been attributed to the internalin operon, which encodes InlA and InlB. Because they bind to receptors on the host cell wall, InlA and InlB are commonly credited with inducing the phagocytosis of L. monocytogenes. Either one is sufficient on its own to induce phagocytosis as long as the needed receptors are present on the host cell (E-cadherin for InlA and the hepatocyte growth factor receptor, HGF-Rc/c-Met, for InlB). The host utilizes the zipper mechanism of phagocytosis, in which receptors on the host bind with ligands on L. monocytogenes sequentially, eventually "zipping" the two membranes together and engulfing L. monocytogenes (Figure 2). In addition to InlA and InlB, Listeriolysin O (LLO) is also capable of inducing phagocytosis. Listeriolysin O is a pore-forming toxin similar to many other cholesterol-dependent cytolysins commonly found in gram-positive bacteria. Using the host cell’s ability to repair a plasma membrane, in which a pore has formed, through membrane internalization, LLO forms enough pores in the membrane to induce membrane internalization, bringing L. monocytogenes into the host (6).

Lysis of Host Vacuole
          Once L. monocytogenes is introduced into the host cell, the bacteria must escape from the vacuole in which it was internalized in order to enter the cytoplasm to undergo growth and replication. Again LLO is the protein responsible for pore formation and the escape of L. monocytogenes from the vacuole (7).
         LLO also activates Ca2+ intracellular channels and channel independent Ca2+ mechanisms. These mechanisms result in host cell responses that have diverse effects on both the host cell and L. monocytogenes, including host cell degranulation, pro-inflammatory mediators, apoptosis, and cytoskeletal rearrangement. L. monocytogenes benefit from Ca2+ influx into the host cell through the production of lipid rafts in the host cell’s membrane, to which more LLO can later bind, and cytokine synthesis. Cytokine synthesis is used by L. monocytogenes to activate IP3R Ca2+ channels through tyrosine phosphorylation and G-protein activation of PLCs (8). The IP3R channels release Ca2+ from the endoplasmic reticulum (9), additionally contributing to the excess of Ca2+ in the cytoplasm (8).
          In addition to LLO, L. monocytogenes utilizes two phospholipase Cs (PI-PLC and PC-PLC, which are capable of lysing the vacuole without LLO) to escape from the vacuole. These PLCs function by lysing the individual phospholipids of the membrane into separate phosphates and ester tails, which are then dissolved in the cytoplasmic water of the host cell (12).

Figure 3: A. Diagram of L. monocytogenes replication. B. Fluorescence microscopy image of L. monocytogenes (red dots) inside host cell. Green streaks behind the red dots are actin filaments. Image from Molecular Biology of the Cell. 4th edition. Fluorescence microscopy image from Theriot and Mitchinson. http://www.microbeworld.org/component/jlibrary/?view=article&id=7627


Actin Nucleation and Motility
          In order for L. monocytogenes to enter the next host cell, they must find a method for motility. ActA, the second gene of the lecithinase operon, is responsible for both actin nucleation and motility (11). It is responsible for the production of ActA protein, a surface protein of L. monocytogenes that induces the production of actin within the host cell, focusing the production at one pole of the bacteria. Additionally, actA creates binding sites on the bacteria for host cell actin to attach to; actin is bound to an NH2 group of the ActA protein (7). Once actin is nucleated at a pole of L. monocytogenes, the Arp2/3 complex is activated and assists in binding the actin filaments together at the tail, making the rotation of the actin capable of propelling the bacteria (Figure 4) (13). However, the nature of actin encourages constant binding and formation at their barbed ends. Therefore, it is necessary to have a regulating protein near the Arp 2/3 complex capable of slowing growth near the junction while not preventing actin formation where it is needed. The enabled/vasodilator-stimulated phosphoproteins (Ena/VASP) bind to actin and delay F-actin capping, promoting the formation of linear actin filaments instead of highly branched ones (2, 14). Through this complex mechanism, L. monocytogenes is able to move not only through the host cell but also into adjacent host cells to repeat the cycle of growth and replication (Figure 3B) (11).
          However, L. monocytogenes does not just stop once it reaches its destination; it stops when there is not longer production of the actin complex, which halts movement (11). This is so because of the relative rigidity of actin, which allows for elastic Brownian ratchet movement to act as the driving force of the cell as more actin is produced (15).

Intercellular Spread
          After L. monocytogenes enters the new host cell, it is engulfed by two layers of membranes, one from the cell from which it came and one from the new cell it enters. Just as in the original host cell, L. monocytogenes must lyse both of these membranes in order to enter the host cell’s cytosol (16). To accomplish this, L. monocytogenes not only uses Listeriolysin O, PI-PLC, and PC-PLC, but also lecithinase, which is only produced after actA produces ActA protein in the lecithinase operon, suggesting there may be a specific need for lecithinase in helping with the destruction of the two host membranes instead of just one (16). Other than the addition of lecithinase, the process is the same as the lysing of the singular cell membrane. From here, the process repeats itself as L. monocytogenes spreads throughout the host.

Virulence Factors


          Of the many genes that contribute to the virulence of L. monocytogenes, five of them are regulated by the prfA sequence in the plcA-prfA operon. Although the virulence proteins that are regulated by the prfA sequence do not always have the same level of expression, they all have higher expression levels when in macrophages compared to epithelial or endothelial cells. Virulence expression also appears to be somewhat dependent upon environmental factors not necessarily specific to the cell. PrfA protein contains a leucine zipper structure that binds to DNA in order to activate the translation of the sequences responsible for coding the virulence factors (17). Although it is unkown what the exact method of prfA activation is (18), it appears to have positive self-regulation (19) and it is clear that a lack of prfA expression lowers the cell’s virulence to almost none and constitutive expression of prfA is not beneficial to L. monocytogenes; proper regulation is needed (18). The genes regulated by prfA are:

Figure 4: Image of the junction between two actin filaments (purple ribbons), held together by the arp 2/3 protein (multicolored). Image courtesy of Science Magazine. http://www.sciencemag.org/content/293/5539/2456/F3.large.jpg

• plcA(17), responsible for PI-PLC production. PI-PLC is capable of hydrolyzing PI and PI-glycan (11). Additionally, PI-PLC is used in the escape of L. monocytogenes from the host phagosomes (12).

• hly(17), responsible for the formation of listeriolysin O, the main factor in classification of L. monocytogenes virulence and a pore-forming toxin that assists in the induction of L. monocytogenes into the host cell and release of it from the phagosomes (11).

• mpl(17), codes for a metalloprotease. Without this gene, about one half of the necessary genes are not coded. L. monocytogenes can survive without LLO if Mpl and PC-PLC are coded for (20).

• actA(17), responsible for everything involving actin (figure 4), including the formation of ActA, the protein responsible for nucleating actin formation at one pole of the bacterial cell and allowing actin to attach to the cell (11).

• plcB(17), responsible for PC-PLC production, which, similarly to PI-PLC, can help the escape of L. monocytogenes from the phagosome, and it is commonly activated during intracellular spread (3).

Other genes that induce virulence that are not regulated by prfA include:

• inlA and inlB, which code for internalin A and internalin B. Interestingly, prfA seems to partially regulate these genes. The proteins help induce the phagocytosis of L. monocytogenes by non-phagocytotic cells, enabling the spread of the bacteria throughout more of the host’s cells (17).

• p60 gene, which codes for p60 protein. Although the exact function of p60 protein is unknown, it is known to be essential for L. monocytogenes virulence and is believed to be responsible for shortening the length of bacterial chains and encouraging the invasion of host cells (21).

• fbpA, a more recently discovered gene, encodes for fbpA protein, which helps L. monocytogenes adhere to host cells, promoting induction. Additionally, it appears to increase the amount of LLO and internalin B, which increase the virulence of the bacteria (22).

The Use of Antibiotics Against Infection


          The use of antibiotics commonly effective against gram-positive bacteria has also been effective against L. monocytogenes. However, L. monocytogenes is starting to go the way of strains of Staphylococcus, and Streptococcus; slowly but surely resistance to these antibiotics is evolving. A single strain of L. monocytogenes was found to have resistance to chloramphenicol, erythromycin, and streptomycin (23). Additionally, species of Listeria closely related to L. monocytogenes but not pathogenic are known to have resistance in many other common antibiotics, such as tetracycline, penicillin G, ampicillin, vancomycin, and gentamycin (4). Most of these resistance factors are found on plasmids capable of interspecies horizontal transfer, so it will not be long before resistance to these antibiotics is no stranger than Streptococcus resistance to them. In fact, many of the resistance genes found in L. monocytogenes were transferred from Streptococcus or Enterococcus (23).
          Specific antibiotic resistance for streptomycin, erythromycin, tetracycline, and chloramphenicol in L. monocytogenes has traced to plasmid pIP811, which is easily transferable among common gram-positive bacteria. The genes in this plasmid were extremely similar to those with the same resistance capacities found in Enterococci or Streptococci; one gene (responsible for chloramphenicol resistance) is even homologous to its counterpart in Enterococci and Streptococci (23).

Figure 5: General skeleton structure of cephalosporin antibiotics. http://goldbook.iupac.org/C00939.html

          L. monocytogenes expresses uniquely high resistance to cephalosporins, which are typically used to treat gram-positive infections of unknown origin due to their strength. The resistance is largely contributed to the gene liaS, which simultaneously increases the susceptibility of the cell to nisin. Conversely, mutant lmo2229 increases cell resistance to nisin while decreasing resistance to cephalosporins (Figure 5). In order to make L. monocytogenes as susceptible to nisin as possible, research is constantly being done on the virulence factors of L. monocytogenes to find weaknesses to be exploited. Recently, the TelA gene has been discovered to be extremely susceptible to both nisin and antibacterial medications. The eventual goal is to implicate the susceptible sequences into L. monocytogenes, lowering the chances of it to cause foodborne illness (24).
          Another important gene discovered in L. monocytogenes that provides the bacteria with significant antibiotic resistance is CesRK. This gene is responsible for the formation of many types of membrane proteins and has a two-part system. Its resistance lies not only in ß-lactam antibiotics but also in its increased resistance to ethanol, which poses the possibility of CesRK as serving as a regulator for alcohol-induced genes within L. monocytogenes. Eventually, this may become a problem, as many disinfectants are alcohols, meaning they will need to accommodate for the weakened effectiveness against L. monocytogenes, unfortunately one of the more dangerous species of bacteria (25).
          Similar to the two-part functioning of CesRK, LisRK also is divided into a two-part system. This time, however, the gene is in charge of osmoregulation of the cell, adapting easily to high levels of osmolarity in the conditions of either its host cell or in the outside environment. As osmolarity is usually caused by a change in salt levels, HtrA assists LisRK greatly when trying to adjust osmolarity with the filament formation (26). Additionally, LisRK can adapt to pH stresses and hydrogen peroxide stresses elegantly without facing too much of a burden from the adjustment (27).

Novel Host Cell Defense Mechanism


          Because of its life cycle and habitat, L. monocytogenes is a tricky bacterium to work with. Its easy spread from one host to another makes it rather difficult to combat, especially because of its location within macrophages of the host (11). For now, a combination of antibiotics seems to work fairly well, but, similarly in fashion to S. aureus, many strains are beginning to develop antibiotic resistances (4). Therefore, it is helpful to investigate methods of defense mechanisms within hosts to analyze where weaknesses within the immune system responses may be (28). Below are some current investigations in which host cell defense mechanisms for L. monocytogenes have been analyzed.

Antimicrobial Peptides (AMPs)
          Antimicrobial peptides are diverse molecules responsible for immunity to a plethora of fungi, viruses, and bacteria. By attacking all sorts of cellular processes, they prevent infection of the host organism, and specific α-defensins have even been observed inhibiting bacterial toxins. In the case of L. monocytogenes, a human AMP, HNP-1, is used to trap the bacteria within the host cell’s vacuole, preventing the proliferation of the species in the host. The AMPs inside human macrophages not only trap the foreign species but also induce higher levels of phagocytosis. The presence of L. monocytogenes induces α-defensin HNP-1 production within the host cell as an immune response to the bacteria’s presence. HNP-1 does not assist with the phagocytosis of L. monocytogenes, but it does help trap them within the vacuoles of the host cell (28). This prevents replication, growth, and spread, as L. monocytogenes needs to be within the host cell’s cytosol to begin these processes (3). The HNP-1 does so by colocalizing with the bacteria within the host cell. Once this occurs, HNP-1 is capable of preventing the release of listeriolysin O from phagosomes and also halts its ability to form channels through the membranes. Hence, HNP-1 traps L. monocytogenes inside the phagosome until the host cell properly deals with the invader. AMPs may be the host cell’s single most effective method of repressing the infection (28).

Autophagy Mediating Pathways
          Although L. monocytogenes is constantly engulfed by macrophages, it is not often that the bacteria are destroyed. Two of the simplest ways to eliminate the bacteria are through destruction of the entire host cell (apoptosis) or the lysing of a certain part (autophagy). Autophagy is insufficient when either the TLR2 or the NOD pathway is not present in the cell. Without the TLR2 or NOD proteins, the ERK pathway is not able to signal the induction of autophagy (29).

Riboflavin Deficiency
          Riboflavin (Vitamin B2) deficiency is common in the same demographics as listeriosis. Riboflavin kinase lyses riboflavin into flavin mononucleotide and flavin adenine dinucleotide, which are essential to proper NADPH oxidase 2 (Nox2) priming. Nox2 contributes significantly to the host cell’s immune response to bacterial infections, therefore making a deficiency of riboflavin detrimental to a host’s ability to fight infections. Specifically, Nox2 depletion causes lowered levels of neutrophils, macrophages, and phagocytes in the host, allowing for L. monocytogenes to survive with fewer threats to its survival (30).

MPYS Pathway Resistance
          When L. monocytogenes enters a host cell, the host cell’s MPYS stimulator is triggered. This stimulation causes Ly6Chi monocytes to be summoned to the infected region using the CCR2 regulatory pathway. In this pathway, MPYS is received by CCR2 (a cytokine receptor) and the signal is sent to the monocytes via MCP-1 or MCP-3 (both ligands of CCR2), causing their release from bone marrow. However, this pathway only induces the release of Ly6Chi; the locating of the site of infection is done through interaction of surface proteins of the monocyte and the host cell, CD11b and ICAM-1 respectively. Once in the infected area, Ly6Chi eliminates the bacteria via phagocytosis (5).

T Helper 17 Cytokines vs. Epstein Barr Virus-Induced 3

Figure 6 reveals data from which it is derived that EBI3 may be used by L. monocytogenes as a protective mechanism, based on its survival in mice positive and negative for the gene (5).


          T helper 17 cells (Th17) are common in the human body and are often responsible for the destruction of foreign pathogenic microbes in the human body, yet they failed to do so when placed in cell culture with L. monocytogenes, unless certain mutations were present in the host cell. In the instances where Th17 was successful in limiting growth of L. monocytogenes, there was a mutation in the host cell’s Epstein Barr Virus-Induced 3 (EBI3) gene (5). EBI3 protein was first found in B-lymphocytes and it is responsible for assisting the regulation of lymphocytic responses (31). It is, therefore, surprising that such a mutation would benefit the immune system of the host. This is apparently due to L. monocytogenes manipulating EBI3 into a defensive utility that cloaks the presence of L. monocytogenes within the cell, therefore tricking the host into believing the pathogen is not present, especially in IL-12 deficient patients. IL-12 is a cytokine with the particular function being the promotion or inhibition of Th17 and Th1 (a cytokine similar to Th17) and is therefore a vital regulator of immune responses within the host. When it is dysfunctional, it is then exceptionally easy for L. monocytogenes to take advantage of EBI3 and use it to hide its presence within the host cell. To combat this method of intracellular survival, researchers are investigating methods to alter EBI3 to make it impossible for L. monocytogenes to utilize it beneficially (5).

References

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 Loureiro, Joseph J; Strasser, Geraldine A; Maly, Ivan V; Chaga, Oleg Y; Cooper, John A; Borisy, Gary G; Gertler, Frank B. Antagonism between Ena/VASP Proteins and Actin Filament Capping
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15. Vazquez-Boland, Jose Antonio; Kocks, Christine; Dramsi, Shaynoor; Ohayon, Helene; Geoffory, Christiane; Mengaud, Jerome; Cossart, Pascale. Nucleotide Sequence of the Lecithinase Operon of Listeria monocytogenes and Possible Role of Lecithinase in Cell-to-Cell Spread. Infection and Immunity. 1992. 60(1): 219-230.


16. Bubert, A; Sokolovic, Z; Chun
, S-K; Papatheodorou, L; Simm, A; Goebel, W. Differential expression of Listeria monocytogenes virulence genes in mammalian host cells. Molecular and General Genetics. 1999. 261: 323-336.


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18. Sheehan, B; Klarsfeld, A; Msadek, T; Cossart, P. Differential activation of virulence gene expression by PrfA, the Listeria monocytogenes virulence regulator. Journal of Bacteriology. 1995. 177(22):6469-6476.


19. Marquis, H; Doshi, V; Portnoy, D A. The broad-range phospholipase C and a metalloprotease mediate listeriolysin O-independent escape of Listeria monocytogenes from a primary vacuole in human epithelial cells. Infection and Immunity. 1995. 63(11): 4531-4534.


20. Bubert, Andreas; Kuhn, Michael; Goebel, Werner; Kohler, Stephan. Structural and Functional Properties of the p60 Proteins from Different Listeria Species. Journal of Bacteriology. 1992. 174(24): 8166-8171.


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