Microbiology — (very) Brief Review

Types of Microorganisms [from smallest up]

**  Viruses — Need electron microscope to visualize

**  Small Bacteria  —  Need electron microscope to visualize
  • Mycoplasma, Chlamydia, Rickettsia
**  Bacteria  —  Light microscopy can visualize
  • Many identified by Gram stain
  • Some need special techniques: e.g. TB (acid-fast stain), Spirochetes (darkfield microscopy)
  • Some too few to see, cause disease by toxins (e.g. tetanus, pertussis)

**  Protozoa  —  Seen by Light microscopy

**  Fungi  —  Seen by Light Microscopy (KOH prep clears other debris)

**  Helminths (worms)
  • Ova & Trophozites seen by Light Microscopy
  • Some adults can be seen grossly (in all senses of the word)

Bacteria   (the focus of “Bugs & Drugs”)

Defined by morphology (shape)
  • Cocci = round balls [singular: Coccus]
  • Bacilli = rods [singular: Bacillus]
Cocci further defined by formation
  • Streptococci = cocci in chains (Greek strepso = “easily twisted”)
  • Staphylococci = cocci in grapelike clusters (Greek staphylo = “grape”)
  • Diplococcus = joined pair of cocci [coffee-bean shaped]
Distinguished by Gram Stain
  • Gram Positive = stains Blue
  • Gram Negative = stains Red

Streptococci  (Gram-Positive Cocci in Chains)

**  Streptococcus pyogenes
  • Skin & Soft Tissue Pathogen
  • Also causes Strep Throat, Scarlet Fever, Rheumatic Fever
  • Group A beta-Hemolytic Strep (GABHS) (Lancefield grouping)
**  Streptococcus pneumoniae (a Diplococcus)
  • a.k.a. “Pneumococcus”
  • Causes the classic lobar pneumonia
  • Most common upper respiratory pathogen (otitis, sinusitis)
  • Does NOT affect skin / soft tissue
**  Streptococcus viridians
  • Normal skin flora, non-pathogenic, contaminant
  • Maybe a pathogen in cases of endocarditis

**  Group A alpha-Hemolytic Strep  —  Not a pathogen

**  Group B Strep
  • a.k.a. Streptococcus agalactiae
  • Normal vaginal flora
  • Causes neonatal sepsis, endometritis
**  Groups C and G Strep
  • Cause sore throat like GABHS
  • Do Not cause Rheumatic Fever
**  Group D Strep
  • a.k.a. Enterococcus faecalis (“Enterococcus”)
  • Causes varied diseases in compromised hosts

Staphylococci  (Gram-Positive Cocci in Grapelike Clusters)

**  Staphyloccus aureus
  • Main Skin & Soft Tissue Pathogen (also colonizes nose)
  • This is what’s meant when you hear “Staph”
**  Coagulase-Negative Staph
  • S. epidermidis, S. saprophyticus, etc.
  • Non-pathogenic skin flora; Usually contaminants
  • May cause IV-catheter-related infection, endocarditis
  • S. saprophyticus may cause UTI

MORE on Micro when we discuss other organ systems