Microbiology: Viable Count of Organisms

Killing Kinetics

• D value (Decimal reduction time) is the time required to reduce the viable population by a factor of 10 (time to kill 90%) under defined conditions.
• D value is used to quantify the probability of an item/area remaining unsterile despite undergoing sterilization process.

Sterilization Methods

• Heat causes loss of structure and function to the fundamental macromolecules of bacterial cells (proteins, DNA, cell membrane).
• Thermal death time is the time it takes to kill all cells at a given temperature.
• Condensation onto cooler surfaces releases latent heat.
• Dry heat vs Moist heat: Most effective after prior evacuation (autoclave).

Media and Growth

• Mannitol Salt Agar is a selective and differential media that differentiates Staphylococcus aureus and Staphylococcus epidermidis based on mannitol fermentation and salt tolerance.
• Blood Agar is an enrichment media (Sheep blood) that may also act as a differential agar, differentiating bacteria based on haemolytic activity.
• Chocolate Agar is a variation of blood agar, heated to >80℃ to lyse red blood cells, providing growth factors and co-enzymes for fastidious organisms like Haemophilus influenzae and Neisseria meningitidis.

Biological Control

• Bacteriophage Therapy uses bacteria-specific viruses that replicate within host cells to produce new phage particles.
• Maggots Therapy uses living chemical factories that produce biologically active molecules, including proteolytic enzymes and antibacterial molecules, to prevent localized colonization and infection.

Other Key Facts

• Presence of bile salts inhibits Gram-positive growth.
• High [salt] inhibits Gram-negative growth.
• Human cells can tolerate a lethal dose of 10 Grays of radiation.