Pathogens for Lecture Exam 1

Clostridium difficile - Gram Stain and Morphology

Gram-positive, large rods (bacilli), arranged in singles or pairs.

Clostridium difficile - Oxygen Requirement

Obligate anaerobe (killed by oxygen; multiplies in zero-oxygen environments like the colon).

Clostridium difficile - Key Biochemical Tests

Catalase negative (-), Indole negative (-), Lecithinase/Lipase negative (-).

Clostridium difficile - Significant Distinguishing Features

Produces tough endospores resistant to heat/disinfectants; motile by peritrichous flagella; produces a horse-barn odor on agar plates.

Clostridium difficile - Medical Relevance and Toxins

Releases Toxin A (enterotoxin) and Toxin B (cytotoxin); major cause of antibiotic-associated diarrhea and pseudomembranous colitis following broad-spectrum antibiotic therapy.

Escherichia coli - Gram Stain and Morphology

Gram-negative, small rods or coccobacilli, arranged in singles or random pairs.

Escherichia coli - Oxygen Requirement

Facultative anaerobe (grows with oxygen, but ferments when oxygen is depleted).

Escherichia coli - Key Biochemical Tests

Catalase positive (+), Oxidase negative (-), Indole positive (+) Lactose fermenter

Escherichia coli - Significant Distinguishing Features

Lactose fermenter (pink colonies on MacConkey agar; metallic green sheen on EMB agar); motile by peritrichous flagella; member of family Enterobacteriaceae.

Escherichia coli - Medical Relevance

Abundant normal flora of large intestine; #1 cause of uncomplicated urinary tract infections (UTIs); toxigenic strains (e.g., O157:H7) cause bloody diarrhea and Hemolytic Uremic Syndrome.

Naegleria fowleri - Classification and Staining

Eukaryotic protozoan (amoeboflagellate); Gram stain is not applicable because protozoa lack peptidoglycan cell walls.

Naegleria fowleri - Three Morphological Stages

1. Trophozoite (amoeboid feeding stage with lobopodia, only form in brain tissue),

2. 2. Flagellate (biflagellated swimming form),

3. 3. Cyst (spherical resting stage in environmental stress).

Naegleria fowleri - Oxygen and Temperature Requirements

Aerobic and thermophilic (thrives in warm freshwater up to 45 degrees C / 113 degrees F).

Naegleria fowleri - Diagnostic Identification and Transmission

Observed as motile trophozoites on CSF wet mount; enters nose while swimming in warm untreated freshwater and travels up the olfactory nerve through the cribriform plate to the brain.

Naegleria fowleri - Medical Relevance

Known as the brain-eating amoeba; causes Primary Amoebic Meningoencephalitis (PAM), a rapid necrotizing brain destruction with greater than 97 percent mortality.

Staphylococcus aureus - Gram Stain and Morphology

Gram-positive, medium cocci, arranged in irregular grape-like clusters.

Staphylococcus aureus - Oxygen Requirement

Facultative anaerobe.

Staphylococcus aureus - Key Biochemical Tests

Catalase positive (+), Coagulase positive (+) (critical test separating it from all other staphylococci), Oxidase negative (-).

Staphylococcus aureus - Significant Distinguishing Features

Halotolerant (grows in 7.5 percent NaCl); ferments mannitol turning Mannitol Salt Agar (MSA) yellow; beta-hemolytic on blood agar; often produces a golden pigment.

Staphylococcus aureus - Medical Relevance

Normal flora of anterior nares in 30 percent of people; causes boils, impetigo, bacteremia, endocarditis, osteomyelitis, and Toxic Shock Syndrome (TSST-1); notorious for MRSA strains.

Staphylococcus saprophyticus - Gram Stain and Morphology

Gram-positive, medium cocci, arranged in clusters, pairs, or singles.

Staphylococcus saprophyticus - Oxygen Requirement

Facultative anaerobe.

Staphylococcus saprophyticus - Key Biochemical Tests

Catalase positive (+), Coagulase negative (-) Oxidase (-) negative

Staphylococcus saprophyticus - Significant Distinguishing Features

Halotolerant on MSA; gamma-hemolytic (non-hemolytic) on sheep blood agar; nonmotile and non-spore-forming.

Staphylococcus saprophyticus - Medical Relevance

uncomplicated UTIs in young, sexually active females.

Treponema pallidum - Morphology and Gram Stain Visibility

Spirochete (tight, regular corkscrew spiral); has a Gram-negative wall architecture but cannot be seen on standard Gram stain because cells are too thin (0.1 to 0.2 micrometers).

Treponema pallidum - How to Visualize in the Lab

Dark-field microscopy (glowing against dark background), fluorescent antibody staining, or silver impregnation stains.

Treponema pallidum - Oxygen Requirement and Culturing

Microaerophilic to host-dependent; cannot be cultured on standard artificial laboratory media.

Treponema pallidum - Significant Distinguishing Features and Diagnostic Tests

Motile via internal periplasmic flagella (axial filaments); diagnosed serologically via screening tests (RPR, VDRL) confirmed by specific treponemal tests (FTA-ABS, TP-PA).

Treponema pallidum - Medical Relevance and Stages

Causative agent of syphilis: Primary (painless chancre), Secondary (rash on palms and soles), Tertiary (gummas, aortitis, neurosyphilis); can cross placenta causing congenital syphilis.

Trichophyton mentagrophytes - Classification and Microscopic Morphology

Eukaryotic fungus (filamentous mold); septate hyphae with spiral coils; abundant grape-like clusters of microconidia and pencil-shaped macroconidia.

Trichophyton mentagrophytes - Oxygen and Growth Characteristics

Obligate aerobe (Strictly dependent on O2)

Trichophyton mentagrophytes - Key Biochemical and Physiological Tests

Urease positive (+) within 2 to 7 days; Hair perforation test positive (+) (erodes wedge-shaped holes into hair shafts).

Trichophyton mentagrophytes - Significant Enzyme and Functional Role

A dermatophyte that produces keratinase to digest and metabolize keratin found in dead skin, hair, and nails.

Trichophyton mentagrophytes - Medical Relevance

Causes cutaneous fungal ringworm infections (tinea): athlete's foot (Tinea pedis), body ringworm (Tinea corporis), jock itch (Tinea cruris), and nail infections (Tinea unguium).