Bacterial-Cells-Structure-Stains-and-Identification.pptx

Bacterial cell identification

1. Bacterial Cells: Structure, Stains, and Identification A comprehensive overview of bacterial cell components, diagnostic staining techniques, and key virulence factors.

2. The Cell Wall: Peptidoglycan and Gram Staining Peptidoglycan Structure The cell wall is composed of a sugar/peptide backbone (NAG and NAM sugars cross-linked by 3-5 amino acids). This structure is the site of action for antibiotics like Penicillin. Gram Positive Thick layer (up to 40 sheets), 50%+ of cell wall. Retains Gram stain, appearing purple . Gram Negative Very few sheets (5-10% of cell wall). Does not retain Gram stain. Unique Cell Walls Mycoplasma: No cell wall; uses sterols for stability. Mycobacteria: Contains mycolic acid; requires Ziehl-Neelsen stain. Chlamydia: Lacks muramic acid.

3. Key Surface Components and Virulence Gram Positive: Lipoteichoic Acid (LTA) Major surface antigen. Induces immune reactions (arthritis, septic shock) and cytokine release. Binds antibodies to activate complement. Gram Negative: Lipopolysaccharide (LPS) Component of the outer membrane. Contains Lipid A (highly toxic, triggers cytokine release) and O antigen (target for antibodies). The Cell Membrane is a phospholipid bilayer present in both Gram (+) and Gram (-) bacteria, hosting electron transport and oxidative phosphorylation.

4. Capsules, Glycocalyx, and Appendages Capsules Distinct, firmly attached gelatinous layer. Prevents phagocytosis. Basis for many vaccines (e.g., conjugated vaccines for S. pneumoniae ). Glycocalyx Irregular, slimy fuzz layer (polysaccharides). Used for adherence to surfaces, such as in S. epidermidis biofilms on catheters. Flagellum Long, protein arms used for bacterial motility. Pili and Fimbria Protein appendages for adherence (ordinary pili) or conjugation (sex pili). Key in E. Coli (UTIs) and Neisseria Gonorrhea .

5. Specialized Structures: Spores, Plasmids, and Ribosomes Spores Dormant state for survival (e.g., Bacillus , Clostridium ). Highly resistant to heat, chemicals, and dehydration due to a keratin-like coat and dipicolinic acid. Plasmids Small, independently replicating DNA molecules. Can carry genes for antibiotic resistance or toxins, and be transferred between bacteria. Ribosomes Site of protein synthesis (50S and 30S subunits). Different from eukaryotic ribosomes, allowing for selective antibiotic action (e.g., Tetracyclines bind 30S).

6. Bacterial Identification: Stains and Pigments Key Staining Techniques Gram Stain: Primary method, but fails for thin bacteria ( Treponema ), those with mycolic acid ( Mycobacteria ), or those lacking cell walls ( Mycoplasma ). Giemsa Stain: Used for Protozoa and intracellular bugs ( Chlamydia , Rickettsia ). Ziehl-Neelsen: "Acid fast" stain for Mycobacterium and Nocardia (due to mycolic acid). Silver Stain: Used for Pneumocystis , Legionella , and H. Pylori . India Ink: Negative stain, primarily for Cryptococcus neoformans capsule ("halos"). Distinct Pigments Staph Aureus: Golden, yellow color. Pseudomonas aeruginosa: Blue-green pigment (pyocyanin). Serratia: Red pigment. Actinomyces: Yellow-orange "sulfur granules."

7. Culture Media: Selective and Differential Growth 1 Blood Agar Non-selective, commonly used to differentiate Streptococcus species (alpha, beta, gamma hemolysis). 2 Thayer-Martin Media (VPN/VCN) Enriched, selective media for Neisseria (gonorrhea/meningitidis), using antibiotics to inhibit other flora. 3 MacConkey's Agar Selective for Gram (-) bacteria (bile salts inhibit Gram (+)). Differential for lactose fermenters (pink colonies). 4 Lowenstein-Jensen Agar Special media for slow-growing Mycobacterium tuberculosis (takes several weeks to form colonies).

8. Bacterial Metabolism and Growth Requirements Obligate Aerobes Require O₂ for ATP generation (respiration). Contain Superoxide Dismutase and Catalase (e.g., Pseudomonas , Mycobacterium ). Obligate Anaerobes Lack key enzymes; use fermentation (no O₂). Often found in gut/mouth flora. Aminoglycosides are ineffective (e.g., Clostridium ). Facultative Anaerobes Can live without O₂, but use it if available (respiration and fermentation). Includes many common bacteria like Staph and E. Coli . Intracellular Bacteria Some bacteria live inside host cells, making them difficult to stain and culture: Obligate Intracellular: Cannot synthesize their own ATP ( Chlamydia ) or depend on host ATP ( Rickettsia ). Facultative Intracellular: Can survive inside macrophages/other cells ( Mycobacterium , Legionella , Salmonella ).

9. Toxins: Endotoxin vs. Exotoxin Endotoxin (LPS) Only in Gram (-) bacteria. Component of the cell wall, released when bacteria die. Lipid A causes toxicity (fever, shock) by triggering TNF and IL-1 release. Classic examples: Meningococcemia, Gram (-) sepsis. Exotoxins Proteins secreted by bacteria (often A-B structure). Cause disease symptoms through various mechanisms: Protein Synthesis Inhibitors ( Diphtheria , Shigella ) Increase Fluid Secretion ( Vibrio Cholera , ETEC ) Neurotoxins ( Clostridium tetani , botulinum ) Lysis of Cell Membranes ( C. perfringens ) Superantigens ( Staph aureus TSST-1)

10. Bacterial Genetics: Mechanisms of DNA Transfer Transformation Conjugation Transduction 1 Transformation Direct uptake of naked DNA from the surrounding environment. 2 Conjugation Transfer of DNA (often plasmids) between bacteria via a sex pilus. 3 Transduction DNA transfer mediated by a bacteriophage (virus that infects bacteria). Genes for several major toxins (e.g., Diphtheria, Cholera) are transferred via lysogeny (a type of transduction).