Listeria Monocytogenes & Gardnerella Vaginalis

• Two non-spore-forming gram-positive rods are Listeria monocytogenes and Gardnerella vaginalis

Listeria monocytogenes

• Causes Meningitis and Sepsis in newborns, pregnant women, and immunosuppressed adults
• Also causes outbreaks of Febrile Gastroenteritis
• Is a major cause of concern for the food industry
• L. monocytogenes is Small Gram-Positive Rod arranged in V- or L-shaped formations similar to corynebacteria
• It has an unusual Tumbling Movement that distinguishes it from corynebacteria, which are nonmotile
• Colonies on Blood Agar plate produce a narrow zone of ẞ-hemolysis, resembling some streptococci
• Listeria grows effectively at cold temperatures; storage of contaminated food in the refrigerator can increase risk of gastroenteritis
• The ability of growth in cold temperature is called "Cold Enhancement"
• L. monocytogenes is actively motile by means of Peritrichous Flagella at room temperature (20 – 25 °C)
• The organism does not synthesize flagella at body temperatures (37 °C)
• At 28°C it is Motile with 1-5 peritrichous flagella.
• At 37°C it exhibits less to no motility.
• On liquid medium it displays "tumbling motility"
• On solid medium it displays "umbrella growth"
• The CAMP Test is used for the Identification of Listeria monocytogenes
• Listeria are catalase positive and produce a small zone of indistinct beta-haemolysis
• Listeria infections primarily occur in two clinical settings:
• Fetus or Newborn (as a result of transmission across the placenta or during delivery)
• Pregnant Women and Immunosuppressed Adults, especially renal transplant patients
• Pregnant women have reduced cell-mediated immunity during the third trimester
• Listeriae is distributed worldwide in animals, plants, and soil
• Transmission happens through ingestion of unpasteurized milk products, undercooked meat, raw vegetables, and contact with domestic farm animals and their feces
• Listeriosis is primarily a foodborne disease associated with eating unpasteurized cheese and delicatessen meats in developed countries like the US
• Following ingestion, the bacteria appear in the colon and can colonize in the female genital tract
• From there, they can infect the fetus if membranes rupture, or infect the neonate during passage through birth canal
• Pathogenesis of Listeria depends on the organism's ability to invade and survive within cells
• Invasion of cells is mediated by internalin made by Listeria and E-cadherin on the surface of human cells
• The ability of Listeria to pass the placenta, enter the meninges, and invade the gastrointestinal tract is dependent on the interaction of internalin and E-cadherin on those tissues
• Upon entering the cell, the bacteria produces listeriolysin, allowing it to escape from the phagosome into the cytoplasm, thereby escaping destruction in the phagosome
• Listeria preferentially grows intracellularly, and the cell-mediated immunity is a more crucial host defense than humoral immunity
• Suppression of cell-mediated immunity predisposes to Listeria infections
• L. monocytogenes can move from cell to cell by means of Actin Rockets/Comet Tails, filaments of actin polymers, and propel bacteria through membrane of one human cell and into another
• Listeria can move using flagella at temperatures of 30°C and below, but at 37°C, it primarily uses actin rockets
• Infection During Pregnancy can result in abortion, premature delivery, or sepsis during the peripartum period
• Newborns Infected at the Time of Delivery can have acute Meningitis 1 to 4 weeks later
• Bacteria reach the meninges via the bloodstream, causing bacteremia
• Infected mothers are either asymptomatic or have an Influenza-Like Illness
• L. monocytogenes infections in immunocompromised adults can present as sepsis or meningitis
• Gastroenteritus caused by L. monocytogenes is characterized by watery diarrhea, fever, headache, myalgias, and abdominal cramps, with little vomiting
• Outbreaks are usually caused by contaminated dairy products, but undercooked meats (e.g., chicken), hot dogs, and ready-to-eat foods (e.g., coleslaw) have also been involved
• Laboratory diagnosis is made primarily by gram stain and culture
• Examination reveals Gram-Positive Rods resembling diphtheroids, forming small, gray colonies with a narrow zone of β-hemolysis on Blood Agar plate, suggesting Listeria presence
• Isolation of Listeria is confirmed by the presence of motile organisms, differentiating them from non-motile corynebacteria
• Identification of L. monocytogenes is made by Sugar Fermentation Tests (L. monocytogenes ferments glucose)
• Treatment of invasive disease, such as meningitis and sepsis consists of Ampicillin with or without Gentamicin, or Trimethoprim-sulfamethoxazole can also be used
• Resistant strains are Rare, and Listeria Gastroenteritis typically does not require treatment
• Prevention is difficult due to the absence of an effective immunization
• Limiting the exposure of pregnant women and immunosuppressed patients to potential sources, such as farm animals, unpasteurized milk products, and raw vegetables is recommended
• Trimethoprim-sulfamethoxazole given to immunocompromised patients to prevent Pneumocystis pneumonia can also prevent listeriosis

Gardnerella vaginalis

• Is the main organism associated with Bacterial Vaginosis
• Bacterial vaginosis is the most common vaginal infection of sexually active women
• Gardnerella vaginalis is a small, facultative, gram-variable rod
• The term "Gram-variable" indicates that some organisms appear purple, while others appear pink in a gram-stained specimen
• Structurally, it has a Gram-Positive Cell Wall, but the wall is thin, and older organisms tend to lose the purple color
• The pathogenesis of bacterial vaginosis remains uncertain
• G. vaginalis is often found in association with Anaerobes, such as Mobiluncus and Prevotella, along with Non-Anaerobes like Mycoplasma hominis and Ureaplasma urealyticum
• Together, these microorganisms cause the symptoms of Bacterial Vaginosis
• Bacterial vaginosis is NOT thought to be transmitted by sexual activity
• It is considered to be a dysbiosis, in which the Lactobacillus found as normal flora in the vagina are replaced by these other organisms
• Bacterial vaginosis is characterized by a malodorous, white, or gray-colored vaginal discharge
• The discharge exhibits a characteristic "fishy" odor
• Inflammatory changes are typically absent
• Mild itching may occur
• Women with bacterial vaginosis have a higher incidence of preterm deliveries, resulting in a higher incidence of morbidity and mortality in their newborn children
• Clue Cells, vaginal epithelial cells covered with bacteria, are an important laboratory finding from a microscopic examination of vaginal discharge
• The "Whiff Test", treating the vaginal discharge with 10% KOH and smelling for a pungent, "fishy" odor, is often positive
• Trichomoniasis, also causing a positive Wiff test, must be ruled out before a diagnosis of bacterial vaginosis can be made
• A pH of greater than 4.5 of vaginal discharge supports a bacterial vaginosis diagnosis
• The drug of choice is Metronidazole
• Treatment of sexual partners is not recommended as it is not considered to be transmissible
• There is no vaccine

Questions cover differentiating Listeria, its growth conditions, identification via the CAMP test, motility, and treatment of listeriosis. Also includes a question about Gardnerella vaginalis and bacterial vaginosis.