Case study microbes

Aspergillus

Category: Fungus; nondimorphic

Skin: Ulceration on skin. Primary cutaneous – starts as skin infection; secondary cutaneous – starts as lung infection

Eye: Rare, occurs w/ wound to eye

Respiratory: Ball of fungal tissue in lung; shortness of breath; pneumonia

Candida albicans

Category: Fungus;
dimorphic.
Yeast commensal -> invasive
pseudohyphae

Skin: Intertrigo, localized rash, yellow/brittle nails

Other Details: Contact, may already be present on skin

Cryptococcus neoformans

Category: Fungus;
dimorphic.
Ubiquitous in bird droppings/soil.

Respiratory: Fever, fatigue, dry cough.
Can go to brain

Other Details: Inhalation from environment.
Yeast with capsule protecting it from phagocytes

Histoplasma capsulatum

Category: Fungus;
dimorphic; in U.S – mostly in Midwest

Skin: Lung -> skin lesions; can also cause meningitis

Respiratory: Granulomas that resemble Tb; chronic lung infection.
Most patients clear on their own

Other Details:Inhalation from environment.
Survives in macrophages.

Haemophilus influenzae

Category: Gram- coccobacillus

Skin: Ear infection

Eye: Conjunctivitis

Respiratory: Pneumonia

Other Details: Increasing antibiotic resistance

Helicobacter pylori

Gram stain: Gram-negative

• Chronic gastritis

• Peptic ulcers

  • Especially duodenal ulcers

  • Also gastric ulcers

Fecal–oral most common

Oral–oral (shared utensils, saliva)

More common in crowded living conditions and early childhood

Listeria monocytogenes

Gram stain: Gram-positive

foodborne, especially from refrigerated or unpasteurized foods:

• Unpasteurized dairy (e.g., soft cheeses)

• Deli meats, hot dogs

• Smoked seafood

• Raw vegetables and coleslaw

• Contaminated refrigerated foods (it grows at 4°C!)

Vertical transmission:

• Transplacental

• During childbirth

Mild, self-limited gastroenteritis

Fever, diarrhea, nausea, myalgias

Neisseria meningitidis

classification - Gram- coccus

blood/neuro condition - Meningitis + petechial rash

infection route - Resp. infection -> blood -> CNS

details - In meningitis belt of Africa, vaccine, capsule, pili, endotoxin, protases

Pseudomonas aeruginosa

Category: Gram- rod

Skin: Swimmer’s ear; “hot-tub rash”;
infection of burned skin

Other Details: Contact w/ environment.
Pyocyanin & pyoverdin siderophores to grab iron;
fimbriae, exoenzymes, exotoxin A;
slime

Shigella

Gram-negative bacteria

• Fecal–oral

• Extremely contagious

• Common sources:

  • Contaminated food/water

  • Daycare centers

  • Overcrowded settings

  • Person-to-person spread (common)

Classic presentation:

• High fever

• Watery diarrhea → rapidly progresses to bloody, mucoid diarrhea

• Tenesmus (painful urge to defecate)

• Crampy abdominal pain

• Dehydration

• Seizures in children (toxin or high fever)

Neisseria gonorrhea

Gram-negative diplococci

• Sexual transmission (vaginal, anal, oral)

• Perinatal transmission → neonatal conjunctivitis

• Highly contagious; often co-transmitted with Chlamydia trachomatis

Men

• Urethritis: burning urination, purulent white/yellow discharge

• Epididymitis

• Proctitis (receptive anal intercourse)

Women

• Cervicitis: mucopurulent discharge, intermenstrual bleeding

• Dysuria (usually without UTIs)

• Pelvic inflammatory disease (PID)

  • Lower abdominal pain

  • Cervical motion tenderness

  • May lead to infertility, ectopic pregnancy

• Pharyngitis (often asymptomatic)

• Conjunctivitis (autoinoculation)

Bacillus anthracis

Category: Bacteria; Gram+ rod endospore former
Skin: Eschar (cutaneous) can lead to sepsis
Respiratory: High mortality rate if untreated
Notes: Inhalation or wound entry. exotoxins

Clostridium botulinum

classification - Gram+ bacillus, endospore former

blood/neuro condition - Botulism (flaccid paralysis: muscle can’t contract)

infection route - Toxin in food

details - A-B toxin (botulinum toxin) blocks release of acetylcholine

Clostridium tetani

classification - Gram+ bacillus, endospore former

blood/neuro condition - Tetanus (spastic paralysis: muscle can’t relax)

infection route - Wound -> blood

details - A-B toxin (tetanus toxin) blocks release of glycine+GABA

Streptococcus mutans

Gram-positive cocci

Transmission

• Person-to-person via saliva

  • Commonly from mother → infant (sharing utensils, kissing)

• Overgrowth occurs with:

  • High-sugar diet

  • Poor oral hygiene

  • Reduced saliva (dry mouth)

. Dental Caries (Cavities)

• Most important role

• Presents with:

  • Tooth sensitivity

  • Pain with cold, heat, or sweets

  • Visible pits, soft spots, or darkened enamel

2. Infective Endocarditis

• Subacute bacterial endocarditis on damaged heart valves

• Especially in:

  • Patients with mitral valve prolapse

  • Rheumatic valve disease

  • Prosthetic valves

• Symptoms:

  • Low-grade fever

  • Fatigue

  • Heart murmur worsening

  • Osler nodes, Janeway lesions, etc. (less common)

Streptococcus pneumoniae

Category: Gram+ coccus

Skin: Acute otitis media (middle ear infection)

Respiratory: Pneumonia, cough, bloody sputum

Other Details: Direct contact; resp. secretions; droplets;
Alpha-hemolysin, autolysin;
capsule, pneumolysin O

Streptococcus pyogenes

classification - Gram+ coccus

blood/neuro condition - Meningitis septicemia, toxic shock syndrome, ARDS, necrotizing fasciitis

infection route - Most common: Pneumonia -> blood

Abdominal infection -> blood

UTI -> blood

details - Septicemia causes a runaway immune reaction leading to organ failure; superantigen overstimulates immune system

Cryptosporidium parvum

Protozoan parasite(protist)

Transmission

• Fecal–oral route

• Infectious oocysts spread through:

  • Contaminated water (most common)

  • Swimming pools, water parks

  • Animal contact (calves, farm animals)

  • Daycare centers

  • Contaminated food

Immunocompetent Individuals

• Watery diarrhea, often large volume

• Cramps

• Nausea

• Low-grade fever

• Self-limited (usually 1–2 weeks)

Immunocompromised (especially HIV/AIDS, CD4 < 100)

• Profuse, chronic, life-threatening watery diarrhea

• Can be 10–20+ liters/day

• Severe dehydration and electrolyte imbalances

• May involve biliary tree → cholangitis, cholecystitis

Entamoeba histolytica

Protozoan parasite (protist)

• Fecal–oral

• Ingestion of cysts in:

  • Contaminated water

  • Contaminated food

  • Unwashed produce

• More common in:

  • Endemic/tropical regions

  • Travelers

  • Men who have sex with men

  • Institutions with poor sanitation

Intestinal Amebiasis

• Gradual onset abdominal pain

• Bloody diarrhea with mucus

• Tenesmus

• Weight loss

• Low-grade fever

• Dysentery tends to be less severe than Shigella but more persistent

Amebic Liver Abscess

• Most common extraintestinal manifestation

• Symptoms:

  • RUQ pain

  • Fever

  • Tender enlarged liver

  • Pain radiates to shoulder

• Abscess fluid:

  • “Anchovy paste” (thick, brownish, without odor)

  • Usually sterile on bacterial culture

Toxoplasma gondii

classification - Protist

blood/neuro condition - Mono-like symptoms, loss of pregnancy, possible behavior change, HIV+ risk of fatal seizure

infection route - Fecal/oral route

details - Main life cycle in cats + birds/rodents, can form cysts in muscle, cross placenta

Fifth disease

Category: Parvovirus B19, ssDNA, no env.

Skin: “slapped cheek” rash spreads face -> trunk/limbs

Respiratory: Cold-like symptoms

Other Details: Respiratory secretions

Hepatitis

virus

• Fatigue

• Nausea/vomiting

• RUQ pain

• Jaundice

• Dark urine, pale stools

• Elevated ALT/AST (ALT > AST in viral hepatitis)

• Hepatomegaly

Herpes simplex virus I

Category: dsDNA, env.

Skin: Cold sore/fever

Other Details: Direct oral contact, latent in nerve ganglia, blister (usually lip)
can spread even if asymptomatic

Herpes simplex virus II

DNA virus, enveloped

transmission

• Sexual contact (genital–genital, genital–oral)

Primary Genital Herpes (First Outbreak)

Often most severe:

• Painful vesicles on erythematous base (genitals, anus)

• Vesicles → ulcers

• Dysuria

• Tender inguinal lymphadenopathy

• Fever, malaise

• Lasts 2–3 weeks

Recurrent Genital Herpes

• Usually milder and shorter

• Triggered by stress, illness, fever, menstruation, immunosuppression

• Tingling/burning prodrome → clusters of vesicles

HIV

enveloped virus RNA 

HIV is transmitted through body fluids containing the virus. The main routes are:

1. Sexual contact (most common worldwide)

   • Vaginal, anal, and less commonly oral sex.

   • Risk is higher with anal sex and in presence of other STIs.

2. Parenteral

   • Sharing needles/syringes (common in intravenous drug use)

   • Accidental needle-stick injuries in healthcare settings

   • Blood transfusions with infected blood (rare now in screened blood supplies)

3. Mother-to-child (vertical) transmission

   • During pregnancy, labor and delivery, or breastfeeding

4. Rare routes

   • Organ transplantation from infected donor

   • Occupational exposure in healthcare

• Routes of Entry

  • Mucosal surfaces: Sexual transmission via genital or rectal mucosa 

Human papillomavirus (HPV)

Category: dsDNA, nonenv.

Skin: Warts

Other Details: Direct contact

Roseola

Category: HHV-6, dsDNA, env.

Skin: Rash after 3-4 days; chest -> limbs; firm raised papules

Respiratory: Runny nose, sore throat, cough, fever > 39.4 C

Other Details: Resp. secretions

rotavirus

Double-stranded RNA virus non-env

Transmission

• Fecal-oral route (most common)

• Contaminated food, water, surfaces

• Highly infectious

Mainly affects children under 5

• Symptoms: Acute watery diarrhea, vomiting, fever, dehydration

• Usually self-limiting but can cause severe dehydration

Ascaris lumbricoides

Nematode (roundworm)

Transmission

• Fecal-oral route (ingestion of eggs from contaminated soil, food, or water)

• Eggs are resistant and can survive in soil for years

Ingestion of embryonated eggs → larvae hatch in small intestine

• Larvae penetrate intestinal wall → bloodstream → lungs

• Ascend respiratory tract → swallowed → return to small intestine

• Mature into adult worms → lay eggs → excreted in feces

Heavy infections → abdominal pain, malnutrition, intestinal obstruction, growth retardation

• Larval migration → transient cough, eosinophilia (Löffler’s syndrome)

hookworm

Nematode (roundworm)

Transmission

• Skin penetration by infective larvae (usually from contaminated soil)

• Fecal contamination of soil is key for larval development

Larvae in soil → penetrate skin (feet commonly)

• Enter bloodstream → lungs → ascend trachea → swallowed

• Mature in small intestine → attach to mucosa → feed on blood

• Eggs passed in feces → hatch in soil → cycle continues

Clinical Features

• Skin: Itchy rash (“ground itch”) at entry site

• Lungs: Cough, mild respiratory symptoms during larval migration

• Intestine: Iron-deficiency anemia, malnutrition, abdominal discomfort

• Heavy infection: Growth retardation in children

Taenia

classification - Tapeworm (cestode)

blood/neuro condition - Brain: neurocysticercosis

infection route - Fecal/oral

details - Rare escape of tapeworm from GI system to blood then brain where it forms cysts