Microbiology

General Lecture Focus

• Topic Overview of major bacterial groups

  • Gram-negative and gram-positive bacteria

  • Phototrophs

  • Archaea and deep-branching bacteria

  • Protists

  • Protozoa

  • Helminths

  • Fungi

• Emphasis on clinically relevant organisms and exam-style recall

  • Format: organism → gram status/group → disease/clinical sign

Gram-Negative Overview

Proteobacteria

• Characteristics

  • Large, diverse phylum

  • Many medically relevant genera

• Example Discussed: Helicobacter pylori

  • Characteristics:

  • Microaerophilic

  • Urease-positive

  • Adapted to acidic stomach environment

  • Clinical Relevance:

  • Causes gastritis and peptic ulcer disease

  • Changed prior assumptions: previously thought to be just stress-related

  • Exam Tip:

  • Know mechanism for acid survival: urease neutralizes stomach acid

  • Understand disease association

Spirochetes

• Morphology:

  • Corkscrew-shaped

  • Axial filament/axial flagella located in the periplasmic space

  • Motility leads to characteristic corkscrew movement

• Key Genera:

  • Borrelia (e.g., Borrelia burgdorferi)

  • Disease: Lyme disease (tick-borne)

  • Treponema (e.g., Treponema pallidum)

  • Disease: syphilis

• Exam Tip:

  • Recall shape, axial filament structure, and major diseases (Lyme disease, syphilis)

Miscellaneous Non-Proteobacteria Gram-Negatives

• Sometimes grouped as “other gram-negatives”

  • Cytophaga/Cytophagia

  • Large, rod-shaped bacteria

  • Common in oral flora; can behave as opportunistic pathogens (e.g., periodontal infections)

Bacteroides

• Major component of gut microbiome (~30% of gut flora)

• Normally, gut flora but can act as opportunistic pathogens:

  • Intra-abdominal infections

  • Abscesses when displaced

Flavobacteria and Related Groups

• Often environmental; unusual reproduction method by budding instead of binary fission

• Common in aquatic environments; may form biofilms and have specialized holdfast attachment structures

• Exam Tip:

  • Recognize common commensals vs. pathogens and their typical habitats (mouth, gut, aquatic)

Phototrophic Bacteria

• Definition:

  • Bacteria capable of photosynthesis; not a single taxon

• Types of photosynthesis:

  • Oxygenic Photosynthesis: Produces O2 (e.g., cyanobacteria)

  • Anoxygenic Photosynthesis: Does not produce O2; utilizes alternative electron donors (e.g., H2S) and pigments

• Pigments vary; bacterial chlorophyll analogs differ from plant chlorophyll

Cyanobacteria

• Ecological Role:

  • Major oxygen producers in aquatic systems

  • Can trigger harmful algal blooms due to nutrient runoff (nitrogen, phosphorus)

• Bloom Implications:

  • Fertilizer use can cause population booms

  • Some strains produce toxins affecting animals and humans

• Exam Tip:

  • Understand ecological role and relationship to algal blooms/toxin production

Gram-Positive Bacteria - High G+C (Actinobacteria)

• Concept:

  • Classification by genomic GC content (>50% G+C)

• Key Genera and Clinical Points:

  • Corynebacterium (C. diphtheriae)

  • Morphology:

    • Gram-positive bacilli, club-shaped; palisade arrangement

  • Disease: diphtheria — respiratory infection with pseudomembrane formation; re-emerging in some areas

  • Growth: fastidious, requires enriched media

  • Gardnerella (G. vaginalis)

  • Gram-variable; associated with bacterial vaginosis (dysbiosis)

  • Clinical signs: discharge, odor; alters epithelial cell appearance on Pap smear (“clue cells”)

  • Mycobacterium (e.g., M. tuberculosis, M. leprae)

  • Characteristics: acid-fast positive due to mycolic acids in the cell wall; classified as gram-positive by phylogeny but stains differently

  • Diseases:

    • Tuberculosis (chronic pulmonary infection with tubercle formation; requires long multi-month therapy, drug resistance increasing)

    • Leprosy

  • Clinical Notes:

    • Important for global morbidity/mortality

    • Long treatment duration due to intracellular/tubercle survival

  • Exam Tip:

    • Know disease associations and unique lab/clinical hints (e.g., palisades for Corynebacterium, clue cells for Gardnerella, acid-fast status for Mycobacterium)

Gram-Positive Bacteria - Low G+C (Firmicutes)

• Group Characteristics:

  • Many are bacilli or cocci

  • Some forms are endospores (notably Bacillus and Clostridium)

• Bacillus genus

  • Large gram-positive bacilli, facultative aerobes, endospore formers; commonly found in soil

  • Examples:

  • Bacillus anthracis — anthrax (spore-mediated, related to soil/animal products)

  • Bacillus cereus — foodborne illness (emetic and diarrheal forms); sometimes implicated in wound infections

  • Morphology:

  • Bacilli are large and form chains; typically larger than Enterobacteriaceae rods under microscopy

Clostridium Genus

• Characteristics:

  • Obligate anaerobic, gram-positive, endospore-forming bacilli; numerous pathogenic species

• Examples:

  • Clostridium tetani — causes tetanus (neurotoxin; spores in soil introduced via puncture wounds; manifests as risus sardonicus/lockjaw)

  • Clostridium botulinum — botulism (botulinum toxin causes flaccid paralysis)

  • Clostridioides difficile — antibiotic-associated colitis (toxin-mediated)

• Clinical Note:

  • Risk associated with environmental persistence of spores and anaerobic conditions in wounds

Staphylococcus and Streptococcus (Major Groups - Review Separately)

• Staphylococcus aureus — Skin/soft tissue infections, MRSA concerns, surgical site infections

• Streptococcus pyogenes — causes strep throat, necrotizing fasciitis, rheumatic fever sequelae

• Exam Tip:

  • Distinguish spore-formers (Bacillus, Clostridium) and know key toxin-mediated diseases and clinical contexts (e.g., puncture wounds for tetanus)

Archaea and Deeply Branching Bacteria

• Archaea

  • Separate domain with distinct lipids in membranes (ether linkages)

  • Often extremophiles (e.g., thermophiles, hyperthermophiles)

  • Some are more closely related to eukaryotes phylogenetically

• Deep-Branching Bacteria

  • Thrive in low-oxygen/ancient-like conditions

  • Include thermophiles

  • Exam Tip:

  • Understand domain-level differences (membrane chemistry, extreme habitats)

Eukaryotic Microorganisms and Pathogens (Overview)

Protists

• Definition:

  • Diverse group of eukaryotic microorganisms; not classified as animals, plants, or fungi

• Protozoa (Animal-like Protists):

  • Unicellular, non-photosynthetic; many are parasitic

  • Examples from Lecture:

  • Entamoeba histolytica — causes amebic dysentery/diarrheal disease

  • Trichomonas vaginalis — sexually transmitted infection (STI)

  • Plasmodium spp. — causes malaria; has a complex life cycle

• Algae: Photosynthetic protists; have implications for algal blooms and toxin production

• Fungus-like Protists (e.g., some oomycetes) — their role in disease is increasing

Helminths

• Definition:

  • Multicellular parasitic worms (e.g., tapeworms, flukes)

  • Lifecycle and pathology differ by species

Fungi

• Increasing clinical significance of fungi:

  • Opportunistic infections

  • Rising antifungal resistance

• Examples from Lecture:

  • Candida albicans — yeast infections, opportunistic

  • Histoplasma capsulatum — systemic fungal infection from environmental exposure

• Exam Tip:

  • Know differences between unicellular vs. multicellular, modes of transmission, typical clinical syndromes

Clinical Patterns and Recurring Themes (Useful for Exam)

• Organism → Group → Staining/Structure → Key Virulence Factors/Unique Lab Hints → Disease

• Example flashcard format:

  • Front: "Gardnerella vaginalis"
    Back: "Gram-variable; part of vaginal microbiota; causes bacterial vaginosis; clue cells on Pap smear; discharge/odor."

  • Front: "Treponema pallidum"
    Back: "Spirochete (axial filament), causes syphilis; diagnosis often by serology."

  • Front: "Mycobacterium tuberculosis"
    Back: "Acid-fast (mycolic acids), causes TB; forms tubercles; requires long multi-month therapy, rising drug resistance."

  • Front: "Clostridium tetani"
    Back: "Anaerobic, spore-forming; tetanus toxin causes muscle spasms/lockjaw; spores introduced by puncture wounds."

  • Front: "Helicobacter pylori"
    Back: "Spiral/curved gram-negative; urease positive; colonizes stomach lining; causes gastritis/peptic ulcers."

Study Actions and Recommended Organization

• Create targeted flashcards for all highlighted organisms (name, gram status, group, key lab trait, disease, primary transmission/clinical sign)

• Create one-page summary tables:

  • Gram-negative: include a breakdown of proteobacteria, spirochetes, Bacteroides, Cytophagia, phototrophic/cyanobacteria with niches and diseases

  • Gram-positive high G+C: Corynebacterium, Gardnerella, Mycobacterium, focusing on key lab clues and diseases

  • Gram-positive low G+C: Bacillus, Clostridium, Staphylococcus, Streptococcus, distinguishing between spore-forming and non-spore-forming and toxin/disease associations

• For pathogens with toxin-mediated diseases, include notes on pathophysiology and clinical context (e.g., C. tetani, C. botulinum, B. anthracis, S. aureus enterotoxin, C. difficile toxins)

• Review ecological notes separately (cyanobacterial blooms, role of fertilizers (N/P) in eutrophication)

Exam Strategy Tips from Lecture Emphasis

• Instructor likely to test recognition/identification (organism → disease) rather than deep mechanistic pathways for every group

• Prioritize memorization of highlighted organisms and their disease associations; retain a few distinctive lab/clinical clues (e.g., palisades for Corynebacterium, clue cells for Gardnerella, acid-fast for Mycobacterium, spirochete axial filament morphology)

• Use active recall (flashcards) and spaced repetition; create image-based cards for morphology (large bacilli vs. E. coli-sized rods, palisades, spirochetes corkscrew)

Quick Reference — Condensed List of High-Priority Organisms from Lecture

• Helicobacter pylori: Stomach ulcers, urease positive, acid survival

• Borrelia burgdorferi: Lyme disease, spirochete

• Treponema pallidum: Syphilis, spirochete

• Cytophagia (Cytophaga): Oral flora, opportunistic

• Bacteroides: Dominant gut flora (~30%), causes intra-abdominal infections when displaced

• Cyanobacteria: Oxygen production, algal blooms, potential toxin production during blooms

• Corynebacterium diphtheriae: Causes diphtheria, presents with pseudomembrane, palisade arrangement

• Gardnerella vaginalis: Causes bacterial vaginosis, clue cells present, discharge/odor

• Mycobacterium tuberculosis: Acid-fast organism, forms tubercles, requires protracted therapy, rising resistance

• Bacillus anthracis: Spore-former, causes anthrax, exposure from soil/animal products

• Bacillus cereus: Causes foodborne illness (emetic/diarrheal)

• Clostridium tetani: Causes tetanus, spores in soil, introduced by puncture wounds (not just rusty nails)

• Clostridium botulinum: Causes botulism, neurotoxin

• Clostridioides difficile: Causes antibiotic-associated colitis

• Staphylococcus aureus: Associated with skin infections, MRSA, surgical site infections

• Streptococcus pyogenes: Causes strep throat, can lead to necrotizing fasciitis