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The Online Textbook of Bacteriology is a general and medical microbiology text and includes discussion of staph, MRSA, strep, Anthrax, E. coli, cholera, tuberculosis, Lyme Disease and other bacterial pathogens.
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Tag words: bacterial growth, antibiotic, chemotherapeutic agent, disinfectant, antiseptic, preservative, control of growth, sterilization, pasteurization.









Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

Bacillus cereus bacteria.Print this Page

Control of Microbial Growth (page 4)

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Chemotherapeutic agents (synthetic antibiotics): antimicrobial agents of synthetic origin useful in the treatment of microbial or viral disease. Examples are sulfonilamides, isoniazid, ethambutol, AZT, nalidixic acid and chloramphenicol. Note that the microbiologist's definition of a chemotherapeutic agent requires that the agent be used for antimicrobial purpose and excludes synthetic agents used for therapy against diseases that are not of microbial origin. Hence, pharmacology distinguishes the microbiologist's chemotherapeutic agent as a "synthetic antibiotic".

Antibiotics: antimicrobial agents produced by microorganisms that kill or inhibit other microorganisms. This is the microbiologist's definition. A more broadened definition of an antibiotic includes any chemical of natural origin (from any type of cell) which has the effect to kill or inhibit the growth of other types cells. Since most clinically-useful antibiotics are produced by microorganisms and are used to kill or inhibit infectious Bacteria, we will follow the classic definition. Note also (above), pharmacologists refer to any antimicrobial chemical used in the treatment of infectious disease as as antibiotic.


Three bacterial colonies growing on this plate secrete antibiotics that diffuse into the medium and inhibit the growth of a mold.

Antibiotics are low molecular-weight (non-protein) molecules produced as secondary metabolites, mainly by microorganisms that live in the soil. Most of these microorganisms form some type of a spore or other dormant cell, and there is thought to be some relationship (besides temporal) between antibiotic production and the processes of sporulation. Among the molds, the notable antibiotic producers are Penicillium and Cephalosporium, which are the main source of the beta-lactam antibiotics (penicillin and its relatives). In the Bacteria, the Actinomycetes, notably Streptomyces species, produce a variety of types of antibiotics including the aminoglycosides (e.g. streptomycin), macrolides (e.g. erythromycin), and the tetracyclines. Endospore-forming Bacillus species produce polypeptide antibiotics such as polymyxin and bacitracin. The table below (Table 4) is a summary of the classes of antibiotics and their properties including their biological sources.

Semisynthetic antibiotics are molecules produced my a microbe that are subsequently modified by an organic chemist to enhance their antimicrobial properties or to render them unique for a pharmaceutical patent.

Table 4. Classes of antibiotics and their properties
Chemical class Examples Biological source Spectrum (effective against) Mode of action
Beta-lactams (penicillins and cephalosporins) Penicillin G, Cephalothin Penicillium notatum and Cephalosporium species  Gram-positive bacteria Inhibits steps in cell wall (peptidoglycan) synthesis and murein assembly
Semisynthetic penicillin Ampicillin, Amoxycillin
Gram-positive and Gram-negative bacteria Inhibits steps in cell wall (peptidoglycan) synthesis and murein assembly
Clavulanic Acid Clavamox is clavulanic acid plus amoxycillin Streptomyces clavuligerus Gram-positive and Gram-negative bacteria Suicide inhibitor of beta-lactamases
Monobactams Aztreonam Chromobacter violaceum Gram-positive and Gram-negative bacteria Inhibits steps in cell wall (peptidoglycan) synthesis and murein assembly
Carboxypenems Imipenem Streptomyces cattleya Gram-positive and Gram-negative bacteria Inhibits steps in cell wall (peptidoglycan) synthesis and murein assembly
Aminoglycosides Streptomycin Streptomyces griseus Gram-positive and Gram-negative bacteria Inhibit translation (protein synthesis)

Gentamicin Micromonospora species Gram-positive and Gram-negative bacteria esp. Pseudomonas Inhibit translation (protein synthesis)
Glycopeptides Vancomycin Streptomyces orientales Gram-positive bacteria, esp. Staphylococcus aureus Inhibits steps in murein (peptidoglycan) biosynthesis and assembly
Lincomycins Clindamycin Streptomyces lincolnensis Gram-positive and Gram-negative bacteria esp. anaerobic Bacteroides Inhibits translation (protein synthesis)
Macrolides Erythromycin Streptomyces erythreus Gram-positive bacteria, Gram-negative bacteria not enterics, Neisseria, Legionella, Mycoplasma Inhibits translation (protein synthesis)
Polypeptides Polymyxin Bacillus polymyxa Gram-negative bacteria Damages cytoplasmic membranes

Bacitracin Bacillus subtilis Gram-positive bacteria Inhibits steps in murein (peptidoglycan) biosynthesis and assembly
Polyenes Amphotericin Streptomyces nodosus Fungi Inactivate membranes containing sterols

Nystatin Streptomyces noursei Fungi (Candida) Inactivate membranes containing sterols
Rifamycins Rifampicin Streptomyces mediterranei Gram-positive and Gram-negative bacteria, Mycobacterium tuberculosis Inhibits transcription (eubacterial RNA polymerase)
Tetracyclines Tetracycline Streptomyces species Gram-positive and Gram-negative bacteria, Rickettsias Inhibit translation (protein synthesis)
Semisynthetic tetracycline Doxycycline
Gram-positive and Gram-negative bacteria, Rickettsias Ehrlichia, Borrelia Inhibit translation (protein synthesis)
Chloramphenicol Chloramphenicol Streptomyces venezuelae Gram-positive and Gram-negative bacteria Inhibits translation (protein synthesis)




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