Common Foodborne Pathogens - VI

Listeria monocytogenes Risk Profile 

Listeria monocytogenes is one of the primary causes of death from foodborne illness, which is a pathogenic, gram-positive, rod-shaped, facultative intracellular bacteria that are motile through its flagella. There are 13 serotypes of L. monocytogenes, including 1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4a, 4ab, 4b, 4c, 4d, 4e, and 7, where serotypes 1/2a, 1/2b, and 4b were associated with the most of foodborne infections. There five other species in the genus Listeria – L. grayi, L. innocua. L. ivanovii, L. seeligeri, and L. welshimeri, but they are not pathogenic to humans, but L. ivanovii is considered pathogenic to ruminants. 

Several L. monocytogenes genes involved in cellular invasion and intracellular parasitism and their functions have been identified, including the pleiotropic regulator of the virulence gene cluster prfA, members of the gene cluster (plcA, hly, mpl, actA, and plcB), and the inl family of invasion genes. Products with roles in phagosomal lysis and escape into the cytoplasm include LLO, a pore-forming toxin encoded by hly, and two C-type phospholipases, a phosphoinositol-specific phospholipase C encoded by plcA and a broad-spectrum phospholipase C encoded by plcB that cleaves phosphatidylcholine (PC-PLC). These enzymes act with LLO to facilitate phagosomal escape and cell-to-cell spread and also may be involved in stimulating intracellular signaling in the eukaryotic target. The mpl gene encodes an enzyme that processes the immature form of PC-PLC into a mature form. Intracellular motility and subsequent cell-to-cell spread are dependent upon the ActA protein, which is essential for polymerization of host F-actin. The recently described inl family of genes encode internalin A and internalin B proteins that are involved in binding and invasion of eukaryotic cells.

L. monocytogenes is a hardy, salt-tolerant bacterium that can survive in temperatures below 1°C and also can grow under psychrotrophic conditions vigorously, unlike many other pathogens. L. monocytogenes is ubiquitous in the moist environments, soil, and decaying vegetation as well as remarkable persistence in food-manufacturing environments.

Sources 
A variety of foods have been implicated with L. monocytogenes, including raw milk, inadequately pasteurized milk, chocolate milk, cheeses (soft cheeses), ice cream, raw vegetables, raw poultry and all kind of meats, fermented raw-meat sausages, hot dogs, and deli meats, and raw and smoked fish and other seafood. Nonetheless, L. monocytogenes can grow in psychotropic conditions, where potential contamination sources include food workers, incoming air, raw materials, and food processing environments. Thus, post-processing contamination at food-contact surfaces poses the greatest threat to product contamination. 

Growth Factors

Temperature:

            Minimum – -0.4°C    Maximum – 45°C     (Optimum 37°C)

pH:

Minimum – 4.4         Maximum – 9.4         (Optimum 7.0)

Water Activity (aW):

Minimum – 0.92       Maximum – -             (Optimum -)

Water Phase Salt:

                        Maximum – 10%      

Disease 

L. monocytogenes is a well-known pathogenic microbe who can infect and replicate within human umbilical vein endothelial cells, and causes central nervous system infections of immunocompromised humans and domesticated animals. 
 

Mortality: 

L. monocytogenes is among the leading causes of death from foodborne infections, and yet it is not a leading cause of foodborne infection. The severe form of the infection can cause a case-fatality rate of 15% to 30%, overall, but if the infection developed into listerial meningitis, the case-fatality rate may increase as high as 70%, and from septicemia, it may go up to 50% overall, further in perinatal/neonatal infections, the mortality rate may be more than 80%.

 

Infective dose: 

The accurate infective dose of L. monocytogenes is not established, where it is believed to be varied with the strain and susceptibility of the host, and the dose-response relationship may affect by the food matrix involved. In cases established against raw or inadequately pasteurized milk has shown that it is likely to be fewer than 1,000 cells for susceptible individuals. However, the infective dose may vary widely and depends on a variety of factors. 

 

Onset: 

Onset can be a relatively short incubation period from a few hours to 2 or 3 days to cause gastroenteritis. However, the severe, invasive form of the infection might take a very long incubation period, before symptoms can be seen which is estimated to vary from 3 days to 3 months. 

 

Complications: 

There are two types of disease caused by L. monocytogenes in humans, which are non-invasive gastrointestinal infections (generally resolves in healthy people) and the much more serious, invasive form of the infection that may cause septicemia and meningitis. Manifestations of L. monocytogenes infection tend to be host-dependent, where people with intact immune systems may cause acute febrile gastroenteritis, which is the less severe form of the disease. However, L. monocytogenes infection tends to be more severe for vulnerable populations. The infection can be grown into a more severe form, where it may result in sepsis and spread to the nervous system, potentially causing meningitis in elderly and immunocompromised people. L. monocytogenes, may cause mild, flu-like symptoms in pregnant women who are disproportionately infected may result in abortion or stillborn of their offspring, and those who born alive may have bacteremia and meningitis. One-third of confirmed cases are usually ending up in abortion or stillbirth due to maternal-fetal L. monocytogenes infections. 

 

Symptoms: 

When infected with L. monocytogenes, healthy people might have mild symptoms or no symptoms, while vulnerable populations may develop fever, muscle aches, nausea and vomiting, and, sometimes, diarrhea. If a more severe form of the infection develops and spreads to the nervous system, the symptoms may further include headache, stiff neck, confusion, loss of balance, and convulsions. 

 

Duration: 

The duration of symptoms generally depends on the health status of the infected person, where symptoms can last from days to several weeks. 

 

Route of entry: 

Oral. 

Pathway: 

L. monocytogenes has unique pathogenesis, where the bacterium can spread directly from one cell to another within the host, instead of traveling interstitially to reach other cells. L. monocytogenes is blood-borne, and once it is attached to the host’s monocytes, macrophages, or polymorphonuclear leukocytes, it starts to reproduce. The phagocytic cell attacks are overcome through a group of proteins on the surface of L. monocytogenes, which further enhance its cell-to-cell spread.

 
Frequency 
Based on a survey conducted in 1997 by the Centers for Disease Control and Prevention (CDC), listeriosis was responsible for approximately 2,500 infections and 500 deaths in the United States annually. L. monocytogenes infections had declined by 36 percent in 2008, compared to the period from 1996 to 1998, but there was a moderate increase in the incidence of L. monocytogenes from 2008 to 2009. More recently, the 2011 CDC report cited above estimated that L. monocytogenes causes 1,591 infections and causes 255 deaths annually, in the United States.  
 
Target Populations 
The main target populations for listeriosis are: 

Pregnant women/fetuses/neonates - perinatal and neonatal infections; 

Persons immunocompromised by i.e., corticosteroids, anticancer drugs, graft suppression therapy, AIDS, 

Cancer patients, particularly leukemic, 

Diabetic, cirrhotic, asthmatic, and ulcerative colitis patients,

The elderly, 

Healthy people.

 
Studies suggest that some of the healthy people are also at risk, because antacids or cimetidine may predispose them to the infection. Furthermore, healthy, uncompromised people could develop the disease, particularly if the food eaten was heavily contaminated with L. monocytogenes. 
 
Diagnosis 

Identification of culture isolated from tissue, blood, cerebrospinal fluid, or another normally sterile site such as placenta or fetus is needed for diagnosis of L. monocytogenes infection. Stool cultures are not informative, since some healthy humans may be intestinal carriers of L. monocytogenes.
 
Food Analysis 
Methods of analyzing foods for purposes of identifying L. monocytogenes are complex and time-consuming. The present FDA method uses a single enrichment broth, buffered Listeria enrichment broth, and requires 24 to 48 hours of enrichment, followed by a variety of agars and, finally, biochemical confirmation. The total time to identification is from 5 to 7 days. Many other enrichment broths, such as UVM broth and Fraser broth, are also included in various protocols. Agars that have been extensively evaluated include Oxford agar, PALCAM, LPM plus esculin, and ferric iron and MOX. New molecular biology techniques have been used to develop various rapid-screening kits for L. monocytogenes. These kits generally rely on ELISA, PCR, and probe-based identification. 
 

 
Reference:
FDA Bad Bug Book, Foodborne Pathogenic Microorganisms and Natural Toxins. Second Edition. 2013
Preventive Controls for Human Foods. 2016
www.cdc.gov
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC107882/