Listeria and S aureus

Overview

  • Focus on two pathogens: Listeria monocytogenes and Staphylococcus aureus

Staphylococcus aureus

Characteristics

  • Catalase: Positive (+)

  • Oxidase: Negative (–)

  • Metabolism: Facultative anaerobe

  • Appearance: Cocci in grape-like clusters

  • Pigmentation: Golden yellow

  • Main Reservoir: Humans

Growth Conditions

  • Optimal Temperature: 30 - 37°C

  • Optimal pH: 6 - 7

  • Growth in Salinity: Can grow at 15% NaCl

  • Water Activity Tolerance: Resistant to low Aw (as low as 0.86 under aerobic conditions)

Toxins Produced

  • Some strains produce:

    • Thermostable nuclease

    • Coagulase

    • Hemolysins

    • Enterotoxin (causes gastroenteritis symptoms)

Human Microflora

  • Commonly found in microflora:

    • S. epidermidis

    • S. aureus

Enterotoxin Characteristics

  • Act at visceral level and can cause:

    • Gastroenteritis (symptoms include nausea, vomiting, abdominal cramps, diarrhea, and headache)

    • Symptoms typically appear 0.5 - 6 hours after ingestion; recovery within 24-48 hours

  • Referred to as “double bucket syndrome”, which is self-limiting.

Toxin Concentration and Stability

  • Minimum concentration for toxicity: 1 ng per gram of food (1-5 µg commonly found in outbreak foods)

  • Heat Resistance: Can withstand 30 minutes of boiling

  • Toxin-producing ability: 105 cfu can produce toxins

Types of Staphylococcal Enterotoxins

  • Various types include SEA, SEB, SEC, SED, SEE, SEG, SEH, SEI, SEJ, SER, SES, SET

Listeria monocytogenes

Characteristics

  • Morphology: Facultatively anaerobic rods

  • Catalase: Positive (+)

  • Oxidase: Negative (–)

  • Fermentation: Produces lactic acid

  • Habitat: Ubiquitous in the environment; found in dairy products, meat, poultry, seafood, fruits, and vegetables

Growth Conditions

  • Optimal Temperature: 30 to 37°C

  • Growth Range: 1 to 45°C

  • pH Range: 5 - 9

  • Salinity: Can tolerate up to 15% NaCl

  • Known for its ability to form biofilms and resilience to adverse conditions

Pathogenic Species

  • L. monocytogenes: Pathogenic to humans

  • L. ivanovii: Pathogenic to animals

Listeriosis

Prevalence and Mortality

  • Incidence: Low prevalence but high mortality rate (20-30% for invasive listeriosis)

  • Carrier State: Asymptomatic carriers exist

  • Infectious Dose: Unknown—potentially around 100 cells

Forms of Listeriosis

  • Non-invasive: Febrile listerial gastroenteritis (symptoms: diarrhea, fever, headache, myalgia).

    • Incubation Period: A few days

  • Invasive: Can lead to meningitis, encephalitis, abortions, neonatal infections, and septicemia.

    • Incubation Period: A few days up to 90 days

Mechanism of Pathogenicity

  • Infection Process:

    • Enters via ingestion

    • Crosses the intestinal barrier

    • Internalized by macrophages

    • Transported to lymph nodes

    • Most are killed in the liver and spleen; survivors may travel to the brain or placenta.

Food Vehicles for Listeria monocytogenes

  • Common in:

    • Meats (including deli products)

    • Poultry

    • Ready-to-eat foods

    • Dairy products (e.g., cheeses)

    • Produce

Discussion Prompt

  • Question: Which is safer to eat – cut melon that has been refrigerated for a week or one that has been freshly cut? Defend your answer.