Part II
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Control of
Microbial Growth
Tim Ho
University of Alberta, Canada
* The materials are mostly based on Dr. Brian Lanoil’s Microb 265
1
Objectives
• Physical agents
• Mechanical removal methods
• Chemical agents
2. Know the strategies on how drugs control the growth of
microorganisms.
3. Understand how do bacteria become resistant to antibiotics.
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1. Know 3 methods of microbial control
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Characteristics of
antimicrobial drugs:
2. Therapeutic dose: the amount of drug required for treatment or the
desired effect.
3. Broad spectrum activity: against a wide variety of pathogens or do
not know the specific bacteria that want to target.
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1. With minimal side effects
4. Chemotherapeutic agents can be synthetic or semi-synthetic.
3
Dilution Susceptibility test
• Each test tube containing different concentrations of drug
- MIC: minimum inhibitory concentration
- MLC: minimum lethal concentration
high [drug]
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Low [drug]
+
+ +
+
+
+
+
-
-
-
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Disk diffusion test
• Kirby-Bauer method
• Measure the diameter of clear zone (no growth) around disks ->
determine MIC and MLC
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• Drug diffuses from disk into agar, establishing concentration
gradient
Large clear zone =
sensitive
No or small clear
zone = resistant
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Image:http://www.biotopics.co.uk/microbes/penici.html
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Disk diffusion test
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Image:http://www.biotopics.co.uk/microbes/penici.html
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How different types of antibiotics
affect cell functions
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Folic acid synthesis inhibitors
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Folic acid synthesis inhibitors
Sulfanilamide:
- Competitive inhibitor of PABA
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Image: Fdardel, 2011
- [PABA] ↑= rate of folic acid biosynthesis ↓
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DNA gyrase inhibitors
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DNA gyrase inhibitors
Ciprofloxacin
Image: Drug Reference - Encyclopedia
Quinolones:
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DNA gyrase: the enzyme
that introduces negative
supercoils into DNA
- inhibit bacterial DNA gyrase
- effective against G- urinary tract infections and respiratory
infections
- (eg. Bacillus anthracis)
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RNA synthesis inhibitors
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RNA synthesis inhibitors
Rifamycin/
Rifampin:
- Not selectively toxic
→ Prokaryotes and
eukaryotes synthesize
nucleic acids in pretty
much the same way
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- block transcription by
binding RNA polymerase
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Cell wall synthesis inhibitors
WHY: ß-lactamases have higher
binding affinity for clavulanic acid
than ampicillin
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- Ampicillin is protected from
lactamases by co-treatment with
clavulanic acid
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Image: Dengler et al. BMC Microbiology 2011 11:16 doi:10.1186/1471-2180-11-16
Cell wall synthesis inhibitors
G + bacteria are more susceptible to ß-lactam antibiotics!!
(transpeptidase)
It breaks ß-lactam rings: antibiotic resistance
- G+ cells: ß-lactamases are located on outside surface
G- cells: ß-lactamases
are in periplasmic space
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Activity is blocked
by binding to
transpeptidases
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Image: Insilico Genomics Lab Technologies. http://insilicogenomics.in/penicillin.asp
Cell wall synthesis inhibitors
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blocks dephosphorylation of
bactoprenol phosphate
blocks transpeptidization
blocks D-Ala
peptidization
Image: Dengler et al. BMC Microbiology 2011 11:16 doi:10.1186/1471-2180-11-16
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Protein synthesis inhibitors
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Protein synthesis inhibitors
Macrolides:
-Binding to the large subunit
ribosome
- First broad-spectrum antibiotics
-Blocking tRNA attachment to
ribosome
- effective against G- and G+ cells
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Tetracyclines:
Aminoglycosides:
-Binding to the small subunit
ribosome
- effective against G- cells
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Cytoplasmic membrane
inhibitors
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Cytoplasmic membrane
inhibitors
Daptomycin:
- Makes pore on
cytoplasmic membrane
- Resistance from
changes in cell
membrane structure
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- Cyclic lipopeptide
- Primarily targets G+
cells (G- cells have extra
outer membrane:
protection)
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Anti-fungal Drugs
• Fungal infections are difficult to treat
- host and pathogen have biological similarity
• Target against chitin (fungal cell wall) mostly
- animals (host) don’t have chitin
• Nystatin: first discovered antifungal antibiotic in 1949 by Hazen and
Brown
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→ drug can harm host at the same time
• Superficial mycoses
- Infections of outer layers of skin
- Treatment (drugs): Miconazole, Nystatin, and Griseofulvin
- Minimizes toxic systemic side effects (e.g. liver damage)
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Antiviral Drugs
• Many drugs are still in development stage
- Structural analogs of purine or pyrimidine bases
- difficulties: viruses use metabolic machinery of the host
• Protease inhibitors: against virus-specific enzymes
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• Mainly target against either RNA or DNA synthesis of viral
pathogen
• Interferons: stimulate production of host anti-viral proteins
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