control of bacterial growth

Control of Bacterial Control of Bacterial GrowthGrowth Antibiotics / Chemotherapy

– History– Properties– Testing– Spectrum of Antimicrobial Action– Modes of Action– Survey of Drugs

Antibiotics Antibiotics

History– Quinine for malaria– Willow bark for treating fever– Paul Ehrlich - staining of bacteria led

to ideas for chemotherapy– Fleming (1928) observed the effect of

Penicillium of on Staphylococcus– Flory & Chain (1940) developed

penicillin and clinically tested it

AntibioticsAntibiotics Peruvian Indians - treat fevers and reduce shivering with

cinchona bark Use " Peruvian bark" was first recorded by the Jesuits in

1633 Countess Anna del Chinchón. was cured of the ague (a

name for malaria the time) in 1638 The Dutch bought the Bolivian seeds from Charles Ledger,

a British botanist, planted them in Java, and came to monopolize the world's supply of quinine for close to 100 years.

A formal chemical synthesis was accomplished in 1944 by American chemists Woodward and W.E. Doering

Since then, several more efficient quinine syntheses have been achieved, but none of them can compete in economic terms with isolation of the alkaloid from natural sources.

Malaria resistant to synthetic but less so to natural The first synthetic organic dye, mauveine, was discovered

by William Henry Perkin in 1856 while he was attempting to synthesize quinine.

AntibioticsAntibiotics 1928 – Fleming discovers penicillin and

isolates a crude form of the chemical 1930’s - Florey and Chain further refine

chemical methods for isolation of penicillin from culture filtrates

WWII – work transferred to Peoria IL – development of submerged culture processes

Post WWII – additional markets for penicillin leads to resistance

AntibioticsAntibiotics

Properties– Selective toxicity (e.g. sulfanilamide

mimics PABA in folic acid synthesis)– Sources

»Microorganisms»Synthetic agents»Plants


Testing– Broth dilution– Agar dilution– Disc diffusion


Broth dilution – MIC - minimal inhibitory concentration

» smallest concentration that stops growth» Successive dilutions inoculated with same

number bacteria» Turbidity measure when compared to control

(could also do dilutions & plate counts)

– MBC - minimal bactericidal concentration» Concentration of antibiotic where cell number is

reduced significantly» Will typically be a higher concentration than MIC


Agar dilution– Dilute drug into agar at varying

concentrations– Can test multiple species of bacteria– Not very quantitative


Disc-Diffusion– Discs with known

concentrations of antibiotics seeded onto “lawn” of bacteria

– Zone of clearing around disk a measure of effectiveness of antibiotic


Spectrum of Antimicrobial Activity– Selectively toxic drugs; uses

differences between prokaryotic and eukaryotic cells

– Broad spectrum – affect both G+ and G-

– Antibiotic effect, e.g. penicillin and Candida albicans


Modes of Action– Bactericidal vs. Bacteriostatic– Cell Wall– Protein Synthesis– Plasma membrane– Nucleic Acid Synthesis– Essential Metabolites


Bactericidal vs. Bacteriostatic– Bactericidal - kills– Bacteriostatic - inhibits growth but

once removed growth can resume


Inhibition of Cell Wall Synthesis– Uniqueness of bacterial cell wall– Prevent peptidoglycan synthesis or

peptide cross-linking from forming– Penicillins & cephalosporins


Inhibition of Protein Synthesis– Bacterial protein synthesis

significantly different than eukaryotic e.g. 70S vs. 80S ribosome or elongation & termination factors

– Amyloglycosides (streptomycin and gentamicin)


Injury of Plasma Membrane– Alteration in permeability– Interference with required

consituents, e.g. sterols in fungal lipid membranes

– Polymixin B (bacteria)– Amphotericin B or miconazole

(fungal)


Essential Metabolites– Para-aminobenzoic acid is an

essential cofactor used by bacteria to synthesize folic acid (a vitamin that functions as a coenzyme in the synthesis of nucleic acid precursors)

– animals ingest folic acid– Sulfanilamide is an analog of PABA


Inhibition of Nucleic Acid Synthesis– Nucleic acid synthesis especially

mRNA and DNA– Rifampin and quinolones– Limited utility because of RNA’s and

DNA’s essential role in both prokaryotic and eukaryotic cells


Survey of Drugs - Cell Wall Synthesis– Penicillins (G, V)– Semisynthetic penicillins (Ampicillin)– Monobactams– Vancomycin - Glycopeptide topical– Cephalosporins– Bacitracin - bacterial origin; topical use– Isoniazid - tuberculosis– Ethambutol - tuberculosis


Survey of Drugs - Protein Synthesis– Amyloglycosides (Streptomycin,

neomycin Gentamicin)– Tetracyclines - Bacteriostatic– Chloramphenicol – Macrolides - Erythromycin -

Bacteriostatic


Survey of Drugs - Plasma Membrane– Polymyxin B - topical; works against

G-


Survey of Drugs - Nucleic Acids– Rifampin– Quinolones– Fluorquinolones


Survey of Drugs - Essential Metabolites– Sulfonamides

control of bacterial growth

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