Bacteria

Cell Structure and Function of Bacteria

Bacteria are prokaryotic microorganisms with diverse structures and functions. Their cell structures contribute to their survival, pathogenicity, and interaction with hosts.

• Flagella as Virulence Factor: Flagella are whip-like appendages that provide motility. Some pathogenic bacteria use flagella to move toward host tissues, aiding in colonization and invasion. Example: Escherichia coli (certain strains), Salmonella.

• Capsule as Virulence Factor: The capsule is a polysaccharide layer outside the cell wall that protects bacteria from phagocytosis and desiccation. Example: Streptococcus pneumoniae, Klebsiella pneumoniae.

• Fimbriae as Virulence Factor: Fimbriae are hair-like structures that help bacteria adhere to surfaces and host cells. Example: Uropathogenic E. coli (UPEC).

Gram-Positive vs. Gram-Negative Bacteria

Bacteria are classified based on their cell wall structure, which affects their staining properties and pathogenicity.

• Gram-Positive (G+): Thick peptidoglycan layer, stains purple. Example: Staphylococcus aureus, Streptococcus pyogenes.

• Gram-Negative (G-): Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), stains pink. Example: Escherichia coli, Pseudomonas aeruginosa.

Acid-Fast Bacteria

• Acid-Fast Positive: Have mycolic acid in cell wall, resist decolorization by acid-alcohol. Example: Mycobacterium tuberculosis.

• Acid-Fast Negative: Lack mycolic acid, do not retain stain after acid-alcohol wash.

Antibiotics: Spectrum and Mechanisms

Antibiotics target specific bacterial structures or functions. Their spectrum refers to the range of bacteria they affect.

• Penicillin, Cephalosporin: Inhibit cell wall synthesis.

• Tetracycline: Inhibits protein synthesis (binds 30S ribosomal subunit).

• Sulfonamides: Inhibit folic acid synthesis.

• Isoniazid: Inhibits mycolic acid synthesis (effective against Mycobacterium).

Horizontal Gene Transfer and Antibiotic Resistance

• Horizontal Gene Transfer (HGT): Movement of genetic material between bacteria, leading to rapid evolution of antibiotic resistance.

• Mechanisms: Transformation, transduction, conjugation.

Exotoxins vs. Endotoxins

• Exotoxins: Proteins secreted by bacteria, often highly toxic. Examples: Neurotoxins (e.g., Clostridium botulinum), cytotoxins, enterotoxins.

• Endotoxins: Lipopolysaccharide (LPS) component of G- outer membrane, released upon cell lysis. Example: Escherichia coli LPS.

Viruses

Basic Structure of Viruses

Viruses are acellular infectious agents with a nucleic acid core and a protein coat (capsid). Some have a lipid envelope.

• Nucleic Acid: DNA or RNA, single- or double-stranded.

• Envelope: Present in some viruses, derived from host membrane.

• Host Specificity: Viruses infect specific hosts or cell types.

Pathogenic DNA and RNA Viruses

• DNA Viruses: Herpes simplex virus, Varicella-zoster virus, Hepatitis B virus.

• RNA Viruses: Influenza virus, HIV, Hepatitis C virus, Rabies virus.

Lytic vs. Lysogenic Cycles (Bacteriophage)

• Lytic Cycle: Virus replicates and lyses host cell.

• Lysogenic Cycle: Viral genome integrates into host DNA (prophage), replicates with host.

• Reverse Transcriptase: Enzyme that synthesizes DNA from RNA (e.g., HIV).

• Integrase: Enzyme that integrates viral DNA into host genome.

Virus Life Cycle and Pathogenicity

• Life cycle stage affects disease severity and persistence.

Types of Viral Infections

• Acute: Rapid onset, short duration (e.g., influenza).

• Chronic: Persistent infection with ongoing virus production (e.g., hepatitis B).

• Latent: Virus remains dormant, can reactivate (e.g., herpesviruses).

Antiviral Therapy

• Acyclovir: Inhibits herpesvirus DNA polymerase.

• Zidovudine (AZT): Inhibits HIV reverse transcriptase.

• Oseltamivir (Tamiflu): Inhibits influenza neuraminidase.

Eukaryotic Microorganisms

Comparison of Eukaryotic Parasites

Eukaryotic parasites include protozoa, fungi, and helminths. They differ in structure, life cycle, and disease epidemiology.

Examples of Pathogenic Eukaryotes

• Protozoan: Plasmodium (malaria), Giardia lamblia (giardiasis).

• Helminths: Ascaris lumbricoides (roundworm), Schistosoma (blood fluke).

• Fungi: Candida albicans (yeast infection), Aspergillus (aspergillosis).

Treatment of Parasitic and Fungal Infections

• Amphotericin B: Antifungal, disrupts fungal cell membrane.

• Ivermectin: Antihelminthic, paralyzes worms.

• Praziquantel: Antihelminthic, increases cell membrane permeability.

• Metronidazole: Antiprotozoal, damages DNA.

Definitive vs. Intermediate Hosts

• Definitive Host: Host in which parasite reaches maturity and reproduces sexually.

• Intermediate Host: Host in which parasite undergoes development but does not reach maturity.

Innate Immunity

Host-Microorganism Interactions

• Normal microbiota protect against pathogens by competing for resources and producing inhibitory substances.

First Line of Defense

• Physical barriers: skin, mucous membranes.

• Chemical barriers: lysozyme, acidic pH.

Second Line of Defense

• Phagocytosis: Engulfment and destruction of microbes by phagocytes.

• Interferon: Proteins that inhibit viral replication.

• Fever: Elevated body temperature inhibits pathogens.

• Inflammation: Localized response to injury or infection.

• Complement Proteins: Enhance phagocytosis, lyse microbes.

Pathogen Evasion of Innate Immunity

• Capsule formation, antigenic variation, inhibition of phagolysosome fusion.

Epidemiology

Reservoirs and Transmission

• Reservoirs: Natural habitats of pathogens (humans, animals, environment).

• Transmission: Direct contact, airborne, vector-borne, fomites.

Drug Spectrum and Selective Toxicity

• Narrow Spectrum: Effective against specific bacteria.

• Broad Spectrum: Effective against a wide range of bacteria.

• Selective Toxicity: Drug targets pathogen with minimal harm to host.

Types of Infections

• Primary: Initial infection.

• Secondary: Follows primary infection.

• Opportunistic: Caused by normally harmless microbes in immunocompromised hosts.

• Latent: Dormant infection.

• Chronic: Long-lasting infection.

• Acute: Rapid onset, short duration.

• Endemic: Constantly present in a population.

• Epidemic: Sudden increase in cases.

• Pandemic: Global epidemic.

• Nosocomial: Hospital-acquired infection.

Emerging/Reemerging Diseases and Bioterrorism

• New or increasing diseases (e.g., COVID-19, Ebola).

• Potential use of pathogens as bioterrorist agents (e.g., Bacillus anthracis).

Microbial Diseases of Organ Systems

Key Concepts

• Identify reservoirs, portals of entry/exit, cell structure, and virulence factors for each disease.

• Recognize symptoms and unique characteristics of diseases affecting skin, soft tissues, eyes, urinary, reproductive, respiratory, and digestive systems.

Methods of Microbial Control

• Sanitization: Reducing microbial load to safe levels.

• Chlorination: Disinfection of water.

• Autoclave: Sterilization using pressurized steam.

• Ethanol: Alcohol-based disinfectant.

• Filtration: Physical removal of microbes.

• UV: Ultraviolet light damages microbial DNA.

• Commercial Canning: Heat sterilization of food.

• Disinfectant: Chemical agents that kill microbes on surfaces.

• Vaccination: Induces adaptive immunity.

Importance of Disease Prevention and Environmental Sanitation

• Prevention reduces disease incidence and spread.

• Sanitizing food, water, and environment is critical for public health.