Principles of Microbial Control

Chapter 7

Introduction to Microbial Control

• Sterilization (destruction of all forms including endospores) , the gold standard of control.

• Suppression (significantly lowering the microbial count)

• Terms used in describing control agents:– Cidal = killed

• Germicide means the killing of cells but not necessarily very resistant forms such as endospore.

– Stasis = inhibit growth• Bateriostasis means the inhibition of growth

– Sepsis = bacterial contamination• Asepsis = the absence of significant contamination. Aseptic

technique used in surgery are practices to minimize contamination from instruments, operating personnel, and patients.

Table 7.1

Definition of Terms: sterilization, disinfection, antisepsis, germicide,

bacteriostasis, asepsis, degerming, sanitation,

Action of Anti-Microbial Agents• Alteration of Membranes

– Membranes control passage of nutrients and wastes. Damage causes leakage which interferes with growth or causes death of the cell.

• Damage to Proteins or Nucleic Acids– Bacteria are ‘little bags of enzymes and small

amounts of nucleic acids.

– Denaturation of proteins causes them to change shape and lose function

– DNA and RNA control the production of proteins. If proteins cannot be made, this is incompatible with life.

Physical Methods Used to Control Microbial Growth

• Pasteurization: quick, mild heating usually of a liquid to kill relevant pathogens or reduce spoilage microbes.– Examples: milk 72 deg C for 15 seconds to kill Q fever

– Milk with no refrigeration: 140 deg C for up to 5 seconds

• Autoclave: 15 psi at 121 C for 15 minutes

• HEPA filtration: High Efficiency Particulate Air Filter

• Freezing: some bacteria survive, some don’t. Ice crystals cause damage to plasma membrane and cell wall.

Figure 7.2

Table 7.3

Table 7.4

Table 7.5 (1 of 2)

Figure 7.3

Figure 7.4

Figure 7.5

Table 7.5 (2 of 2)

Conditions Influencing Microbial Growth

• Pseudomonas: gram negative rod, ubiquitous, antibiotic resistance, detergent resistance.

• Mycobacterium tuberculosis: waxy cell wall, resists drying

• Gram negative bacteria: more resistant to disinfectants and antiseptics

• Non-enveloped viruses: more resistant than enveloped

• Others: endospores, protozoan cysts (Giardia and Cryptosporidium)

Figure 7.6 - Overview (1 of 4)

Figure 7.11

Table 7.7

Chemical Methods of Microbial Control

• Organisms resistance to chemical biocides vary groups. Fig 7.11

• Chlorhexidine (biguanide group). Damages plasma membranes so good for most vegetative bacteria. Uses as a surgery scrub. Often combined with ethanol and/or detergent.

• Halogens (iodine and chlorine) . Halogens are very reactive as a group.– Iodine (I2) and as Tincture ( with alcohol). Inactivates enzymes,

also Betadine (iodophor)– Chlorine(Cl2), Household bleach ( Sodium hypochlorite, NaOCl),

and other forms. Oxidizing agent. Very inexpensive. Not known exactly how it kills.• Drinking water is usually treated with small amounts of chlorine.

Chemical Methods of Microbial Control• Ethylene Oxide: a gaseous chemical sterilizer

• Denatures proteins. Requires 4-18 exposure in a chamber.• Sterilizes blood bags for transfusion and other plastics.

• Alcohols. Use 60-95%. Denatures proteins and dissolves lipids. Table 7.6• Not effective against endospores and non enveloped virus.• Not too effective on wounds

• Surface Active Agents. • Soap: removes bacteria through scrubbing. • Degerming agents: Some have substances added that

inhibit bacteria, so labeled ‘antimicrobial’.• Quaternary ammonium Compounds (Quats) fig 7.9. Disrupt

plasma membranes and cytoplasmic constituents leak out. Popular brands: Zephiran and Cepacol.

• Oxidizing Agents (peroxygens). Ex.: Hydrogen Peroxide. Useful to disinfect inanimate objects.• Inactivated in wounds due to presence of tissue catalase

(breaks down H2O2 to water and O2).

Table 7.6

Benzalkonium Chloride: a Quaternary Ammonium

Compound (Quat)

Figure 7.10

Figure 7.8

Heavy Metal Antimicrobial Action

Drinking Colloidal Silver Will Turn You Blue!