Nervous and Digestive System Infections: Microbiology Study Notes

Other Nervous System Infections

Fungal Infections of the Central Nervous System

Fungal infections of the CNS are rare but can be severe, especially in immunocompromised individuals. Only a few fungi are primary pathogens capable of causing disease in healthy hosts.

• Fungal spores are ubiquitous in the environment (dirt, air, water).

• Prevention of all fungal diseases is nearly impossible due to widespread spores.

• Immunocompromised patients are at higher risk for CNS fungal infections.

Cryptococcosis

Cryptococcosis is a fungal infection primarily affecting immunocompromised individuals, notably those with HIV/AIDS.

• Etiological agent: Cryptococcus neoformans (yeast-like fungus)

• Forms tough, resistant spores

• Transmission: Inhalation of aerosolized bird droppings (e.g., pigeons)

• Mechanism: Fungus enters lungs, invades macrophages, migrates to lymphatic system, can infect blood and invade CNS

Causative agent	Cryptococcus neoformans and C. gattii fungi
Epidemiology	Over 1 million cases annually, mostly in immunocompromised patients; high mortality in sub-Saharan Africa
Transmission	Inhalation of spores from bird droppings
Signs & symptoms	Cough, flu-like symptoms, fever, headache, agitation, seizures
Diagnosis	Cultures, microscopic methods, antigen detection in CSF

Protozoan Nervous System Infections

Protozoans are single-celled eukaryotic parasites with complex life cycles. Some flagellated and amoeboid protozoans cause serious neurological symptoms in humans.

• Facultative anaerobes

• Reproduce sexually or asexually depending on life stage

African Sleeping Sickness

African sleeping sickness is a tropical disease found in Africa, caused by Trypanosoma brucei.

• Subspecies: T. brucei rhodesiense (acute, often fatal), T. brucei gambiense (chronic, 98% of cases)

• Transmission: Tsetse fly bite

• Symptoms: Chancre at bite site, fever, headache, swollen lymph nodes, sleep disturbances

• Progression: Parasite enters blood, crosses blood-brain barrier, infects CNS

• Mechanism: Direct cytotoxic effects, strong immune reaction can damage host neurons

• Complications: Fatal if untreated

Primary Amoebic Meningoencephalitis (PAM)

PAM is a rare but almost always fatal CNS infection caused by Naegleria fowleri, a thermophilic amoeba.

• Transmission: Swimming in warm, stagnant water; use of nonboiled tap water in neti pots

• Mechanism: Amoeba enters nasal passages, burrows into mucosa, travels up olfactory nerves to brain

• Symptoms: Intense headache, sore throat, vomiting, fever, stiff neck, seizures, hallucinations, coma

• Progression: Death usually occurs within 1-2 weeks

• Complications: Postmortem shows hundreds of trophozoites in brain; CSF filled with protozoans and dead WBCs

• Other agents: Acanthamoeba spp., Balamuthia mandrillaris (similar transmission and symptoms, reside in host for 3-24 months before symptoms, almost always fatal)

Toxoplasmosis

Toxoplasmosis is a protozoan infection caused by Toxoplasma gondii, with cats as the definitive host.

• Transmission: Cat feces, contaminated water, undercooked meat, congenital

• Mechanism: Oocysts consumed, develop into tachyzoites, invade tissues; can cross placenta in pregnant women

• Prevention: Immunocompromised and pregnant women should avoid litter boxes

• Symptoms: Usually asymptomatic; in immunocompromised, cysts rupture causing encephalitis (confusion, headache, fever); in pregnancy, miscarriage or stillbirth

• Progression: Untreated cases may result in seizures, psychiatric symptoms, coma, death

Causative agent	Toxoplasma gondii, a protozoan
Epidemiology	Up to 23% of US population exposed; mostly asymptomatic; severe disease in immunocompromised and pregnancy
Transmission	Undercooked meat, cat feces, congenital
Signs & symptoms	Asymptomatic or flu-like; severe cases: encephalitis, miscarriage
Diagnosis	Serology, PCR, ELISA

Transmissible Spongiform Encephalopathy (TSE)

TSEs are fatal neurodegenerative diseases caused by prions (infectious proteins).

• Etiological agent: Prions

• Transmission: vCJD from BSE-contaminated beef; Creutzfeldt-Jakob disease is spontaneous

• Symptoms: Memory loss, difficulty speaking, tremors, death

• Progression: Brain tissue develops sponge-like appearance

• Mechanism: PrPC encounters abnormal PrPSC, becomes infectious, transforms more PrPC, prions clump and kill neurons, dead cell pockets cause sponge-like brain tissue

Causative agent	Prions
Epidemiology	Median age of death for CJD is 68 years; vCJD median age is 28 years; rare in US
Transmission	vCJD: contaminated beef; CJD: spontaneous, genetic, or iatrogenic
Signs & symptoms	Rapid dementia, memory loss, speech impairment, ataxia
Diagnosis	Clinical history, EEG, MRI, brain biopsy

Digestive System Infections

Digestive System Anatomy and Defenses

The digestive system includes the gastrointestinal (GI) tract and accessory organs, and is a major entry portal for pathogens.

• GI tract: Mouth to anus; upper GI (mouth, pharynx, esophagus, stomach), lower GI (small/large intestines, rectum, anus)

• Accessory organs: Salivary glands, liver, gallbladder, pancreas

• Lymphatic tissues: Tonsils, appendix, Peyer's patches

Upper GI Tract

• Food enters at mouth; salivary glands secrete enzymes for lipid/carbohydrate digestion

• Stomach mixes food with gastric juices, begins protein digestion, acidic environment limits microbial growth

• Food and gastric juices form chyme

Lower GI Tract

• Chyme enters small intestine; bile (from liver, stored in gallbladder) aids fat digestion

• Bile salts inhibit growth of many bacteria, especially Gram-positive

• Pancreas secretes digestive substances

• Liver stores nutrients, metabolizes drugs/toxins, receives nutrient-rich blood from small intestine

• Large intestine absorbs water and water-soluble vitamins; undigested matter excreted as feces

Digestive System Defenses

• Lysozyme in saliva

• Acidity of gastric juices

• Mucosa-associated lymphoid tissue (MALT)

• Specific immune cells and lymphatic tissue

Digestive System Microbiome

The digestive tract hosts the most diverse microbiome in the body, with thousands of species.

• Mouth: Streptococcus species most common; teeth colonized by Streptococcus, Neisseria, Fusobacterium, Actinomyces

• Feces: Rich in Bacteroides species; fungi (e.g., Candida) and some protozoa present but bacteria predominate

• Roles: Compete for nutrients, excrete antimicrobials, assist digestion, produce nutrients (e.g., Escherichia coli produces vitamin K), impact metabolism, obesity, immune responses

Gastrointestinal Infection Symptoms and Diagnostic Tools

GI infections can cause a range of symptoms and are diagnosed using various clinical and laboratory methods.

• Diarrhea: Frequent passing of loose or watery stool

• Enteritis: Inflammation of intestines

• Gastritis: Inflammation of stomach

• Gastroenteritis: Inflammation of stomach and intestines

• Dysentery: Diarrhea with pain, blood, and/or mucus

• Dehydration: Excessive loss of body fluid

• Hypovolemic shock: Low blood volume, can lead to organ failure

Transmission and Epidemiology

• Most GI pathogens transmit via the fecal-oral route (contaminated food/water)

• Hand washing, sanitation, and food safety regulations reduce transmission

• CDC estimates 1 in 6 Americans suffer foodborne illness yearly; higher prevalence in developing nations

• Diarrhea and dysentery kill about 2.2 million people annually, mostly children under five in developing countries

Diagnostic Tools

• Fecal sample collection and culture on selective/differential media (e.g., MacConkey agar)

• Molecular diagnostics and microscopic examination for parasites/eggs

• Endoscopy (upper GI: via mouth; lower GI: colonoscopy via anus) to observe tissue damage