Topic 4

• Gram staining


  A common technique used to differentiate two large groups of bacteria based on their different cell wall constituents
• Gram staining procedure

  1. Distinguishes between Gram positive and Gram negative groups by coloring these cells red or violet

  2. Gram positive bacteria stain violet/blue due to the presence of a thick layer of peptidoglycan in their cell walls, which retains the crystal violet these cells are stained with

  3. Gram negative bacteria stain red/pink, which is attributed to a thinner peptidoglycan wall, which does not retain the crystal violet during the de-coloring process
• Antimicrobial chemotherapy


  Based on the principle of selective toxicity, which implies that a compound is harmful to a microorganism but less damaging to its host
• Properties of drugs used in antimicrobial therapy

  • Antimetabolites
  • Inhibit cell wall biosynthesis
  • Inhibit protein synthesis
  • Inhibit nucleic acid synthesis
  • Alter or inhibit cell membrane permeability or transport
• Drug resistance


  Occurs when microbes survive and grow in the presence of a drug that normally kills or inhibits the microbe's growth
• Types of drug resistance

  • MRSA (Methicillin-Resistant Staphylococcus aureus)
  • VRE (Vancomycin-Resistant Enterococci)
• Causes of antimicrobial drug resistance

  • Selective pressure
  • Mutation
  • Gene transfer
  • Inappropriate drug use
  • Inadequate diagnosis
  • Hospital use drugs
• Chlamydia trachomatis


  • Gram-negative bacterium
  • Different types cause different diseases, most common strains cause disease in the genital tract, other strains cause trachoma and conjunctivitis
• Escherichia coli


  • Motile, member of the Enterobacteriaceae, Gram negative, facultative anaerobe
  • Causes urinary tract infections (UTIs), neonatal sepsis and meningitis, and diarrheal diseases
  • All strains have endotoxin and constitutively produce common pili that adhere to colon cells
• Haemophilus influenzae


  • Gram-negative, fastidious, pleomorphic rod
  • Most strains are opportunistic pathogens
  • Transmitted via respiratory droplets by direct contact
  • In infants and young children, H. influenzae type b (Hib) causes bacteremia, pneumonia, epiglottitis and acute bacterial meningitis
  • Causes exacerbations of chronic bronchitis in patients with chronic obstructive pulmonary disease (COPD)
• Pseudomonas aeruginosa


  • Gram-negative rod with polar flagella, aerobe
  • Blue-green pus is a classic sign of P. aeruginosa cellulitis in burn patients
  • Frequent cause of nosocomial infections, with pneumonia in cystic fibrosis (CF), neutropenia, or ventilated patients
  • The capsule inhibits phagocytic uptake, pili also aid in adherence
• Staphylococcus aureus


  • Gram-positive coccus found in grapelike clusters
  • Colonizes in the nasal passages mucosa and can be found on the skin
  • Transmitted by direct contact/fomites (e.g., bedrails)
  • Common cause of skin infections including abscesses, respiratory infections such as sinusitis, and food poisoning
• Streptococcus pyogenes


  • Gram-positive coccus
  • Colonizes skin and upper respiratory tract mucosa
  • Survives on hard surfaces; spread by fomites, respiratory droplets, and direct contact
  • Causes pharyngitis, impetigo, cellulitis (inflammation of subcutaneous connective tissue), scarlet fever