Environmental Factors that Influence Microbes

Including Antimicrobial Agents

Temperature

• Psychrophile – staphylococcus aureus & Listeria monocytogenes ( food-borne illness)

• Mesophiles (20 -40°C). Human body is 37°C.• Thermophile (45 -80°C). - Thermus aquaticus Taq polymerase (enzyme)

• pH, Gases & osmotic pressure

Microbial Growth

Exponential Growth

Growth Curve

Control of Microbial Growth

Physical, Chemical, vs, Mechanical methods of microbial control

Important Terms:• Disinfection:

– The destruction or removal of vegetative pathogens (not bacterial endospores).

• Sterilization:– The complete removal or destruction of all viable microorganisms. Used on

inanimate objects only• Antisepsis:

– Chemicals applied to body surfaces to destroy or inhibit vegetative pathogens.

• Biocide/germicide:

– Kills microbes• Bacteriostasis:

– Inhibiting, not killing, microbes

Principles of Effective Disinfection

• Concentration of disinfectant• Organic matter• pH• Time

Figure 7.2

An Autoclave

What is Microbial Death?

• The permanent loss of reproductive capability, even under the optimum growth conditions.

How Antimicrobial Agents Work…

There are 4 main modes of action:

4 Targets -

1. cell wall synthesis or integrity

2. cell membrane synthesis or integrity

3. protein or nucleic acid synthesis

4. protein structure

Cell Wall Interference

• Purpose of cell wall:• Some chemical agents can:

– block its synthesis• Most common agents prevent cross-linkage of NAM subunits• Ex: penicillin

– digest the cell wall or – break down it’s surface

• Ex: detergents and alcohol

• Bacteria have weakened cell walls and eventually lyse

Figure 10.3c-e Bacterial cell wall synthesis and the inhibitory effects of beta-lactams on it.

New cross-linksinhibited bybeta-lactam

Previously formed cross-link

Methicillin (semisynthetic)

Carbapenems

Imipenem (semisynthetic)

Growth

Beta-lactam interferes with the linking enzymes, and NAM subunits remain unattached to their neighbors. However, the cell continues to grow as it adds more NAG and NAM subunits.

The cell bursts from osmotic pressurebecause the integrity of peptidoglycan is not maintained.

β-lactam ring

Penicillin G (natural)

Cephalosporins

Cephalothin (natural)

Inhibitors of Cell Wall Synthesis

• Penicillin– Natural penicillins– Semisynthetic penicillins– Extended-spectrum penicillins

Polypeptide antibiotics Bacitracin

Topical application Against gram-positives

Vancomycin Glycopeptide Important "last line" against antibiotic-resistant

S. aureus

Surfactants on cell membrane

Protein or Nucleic Acid synthesis interference

• Certain drugs/chemicals can bind to the ribosomes in a way that stops peptide bonds from forming.

• Some agents bind irreversibly to DNA, preventing both transcription and translation, whereas others are mutagenic agents.

Inhibition of Protein Synthesis by Antibiotics

Figure 20.4

Mechanisms of Antimicrobial Action

• Inhibition of Protein Synthesis– Prokaryotic ribosomes are 70S (30S and 50S)– Eukaryotic ribosomes are 80S (40S and 60S)– Drugs can selectively target translation– Mitochondria of animals and humans contain 70S

ribosomes • Can be harmful

Mechanisms of Antimicrobial Action• Inhibition of Nucleic Acid Synthesis

– Several drugs block DNA replication or RNA transcription

– Drugs often affect both eukaryotic and prokaryotic cells

– Not normally used to treat infections – Used primarily in research and perhaps to slow

cancer cell replication

Protein structure

Radiation as a Microbial Control Agent