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بسم الله الرحمن الرحيم. Antimicrobial Chemotherapy to Control Microorganisms. 2013-2014. Diagnostic Medical Microbiology-Laboratory Manual. Antimicrobial Control Agents.

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antimicrobial control agents
Antimicrobial Control Agents
  • Usually, microbial controls are used to avoid contamination of pure cultures, prevent infection, or treat existing diseases.
  • A microbiocidal effectkills microorganisms.
  • A microbiostatic effectprevents the reproduction of microorganisms.
  • Antisepticsare chemicals used on living tissues to inhibit the growth of microorganisms.
  • Disinfectants are chemicals used on nonliving surfaces to inhibit the growth of microorganisms.
  • Chemotherapeutic agentsare chemicals used to destroy or inhibit the growth of microorganisms in living tissues.22
antimicrobial chemotherapeutic agents
Antimicrobial Chemotherapeutic Agents
  • Antimicrobial Chemotherapy :is the use of chemicals to inhibit or kill microorganisms in the host.
  • Selective Toxicity:This means that the agent used must inhibit or kill the microorganism without seriously harming the host.

Based on their origin, there are 2 general classes of antimicrobial chemotherapeutic agents:

Antibiotics:substances produced as metabolic products of one microorganism which inhibit or kill other microorganisms.

Antimicrobial Chemotherapeutic Chemicals:chemicals synthesized in the laboratory which can be used therapeutically on microorganisms.


Antimicrobial agents are :

  • Cidalin action: they kill microorganisms.
  • Staticinaction: they inhibit microbial growth long enough for the body's own defenses to remove the organisms.
  • Antimicrobial agents also vary in their spectrum:
  • Broad spectrum :Drugs which are effective against a variety of both gram-positive and gram-negative bacteria.
  • Narrow spectrum :Drugs which are effective against just gram-positive bacteria, just gram negative bacteria, or only a few species are termed.
  • If a choice is available, a narrow spectrum is preferable since it will cause less destruction to the body's normal flora. In fact, indiscriminate use of broad spectrum antibiotics can lead to super-infection by opportunistic microorganisms.

Several tests may be used to tell a physician which antimicrobial agent is most likely to combat a specific pathogen:

  • 1. Tube dilution tests:
  • A series of culture tubes are prepared, each containing a liquid medium and a different concentration of a chemotherapeutic agent. The tubes are then inoculated with the test organism and incubated for 18-24 hours at 37oC. After incubation, the tubes are examined for turbidity (growth).
  • Minimum Inhibitory Concentration (MIC):
  • Is the lowest concentration of chemotherapeutic agent capable of preventing growth of the test organism.
  • Minimum Bactericidal Concentration (MBC):
  • Is the lowest concentration of the chemotherapeutic agent that allows less than 0.1% of the inoculum to survive , also called “ Minimum Lethal Concentration (MLC)”

2. The agar diffusion test (Bauer-Kirby test):

  • In this test, the in vitro response of bacteria to a standardized antibiotic-containing disc has been correlated with the clinical response of patients given that drug.
  • In the development of this method, a single high-potency disc of each chosen chemotherapeutic agent was used.
  • Zones of growth inhibition surrounding each type of disc were correlated with the minimum inhibitory concentrations of each antimicrobial agent (as determined by the tube dilution test).
  • The MIC for each agent was then compared to the usually-attained blood level in the patient with adequate dosage. Categories of "Resistant,“ "Intermediate," and "Sensitive" were then established.
  • 3. Epsilometer test:
  • It’s a new technique for direct detection of MIC, a graduated increasing concentration of the antibiotic is fixed along a rectangular plastic test strip which is applied to the surface of an inoculated agar plate, after over night incubation a tear drop shaped inhibition zone is seen.
  • The zone edge intersect the graded test strip at the MIC of the antimicrobial.
effects of combinations of drugs
Effects of Combinations of Drugs
  • Additive (indifferent) effect: the activity of two drugs in combination is equal to the sum (or a partial sum) of their independent activity when studied separately.
  • Synergistic effect: the activity of two drugs in combination is greater to the sum of their independent activity when studied separately.
  • Antagonistic effect: the activity of two drugs in combination is less to the sum (or a partial sum) of their independent activity when studied separately.
haw to choose antimicrobial drugs
Haw To Choose Antimicrobial Drugs?
  • Narrow spectrum OR broad spectrum.
  • Old age OR child.
  • Male OR female.
  • Pregnant OR lactating women.
  • In patient OR out patient.
  • Type of microorganisms.
  • Site of infection.

Examples of commonly used antimicrobial chemotherapeutic agents arranged according to their mode of action:

  • Antimicrobial agents that inhibit peptidoglycan synthesis.
  • Inhibition of peptidoglycan synthesis in actively-dividing bacteria results in osmotic lysis.
  • Penicillins (produced by the mold Penicillium)
  • There are several classes of Penicillins:
  • 1. Natural Penicillins:
  • Are highly effective against gram-positive bacteria (and a very few gram negative bacteria) but are inactivated by the bacterial enzyme penicillinase. Examples include PenicillinG and V.

2. Semisynthetic Penicillins :

Are effective against gram-positive bacteria but are not inactivated by penicillinase. Example Methicillin.

3. Semisynthetic broad spectrum Penicillins :

Areeffective against a variety of gram-positive and gram-negative bacteria

but are inactivated by penicillinase. Example Ampicillin.

4.Semisynthetic broad spectrum penicillins combined with beta lactamase inhibitors such as clavulanic acid and sulbactam :

Although the clavulanic acid and sulbactam have no antimicrobial action of

their own, they inhibits penicillinase thus protecting the penicillin from

degradation. Example Augmentin (Amoxicillin plus Clavulanic acid).


B. Cephalosporins (produced by the mold Cephalosporium):

Cephalosporins are effective against a variety of gram positive and gram-negative bacteria and are resistant to penicillinase (although some may be inactivated by other beta-lactamase enzymes similar to penicillinase).

Four "generations" of cephalosporins have been developed over the years in an attempt to counter bacterial resistance :

1.First generation cephalosporins :


2.Second generation cephalosporins :


3.Third generation cephalosporins :


4.Fourth generation cephalosporins :

Cefepime (Maxipime).


c. Monobactems: They work only against Gram-negative bacteria, beta lactam antibiotics resistant to beta lactamase. The only commercially available monobactam antibiotic is Aztreonam.

d. Vancomycin (produced by the bacterium Streptomyces):

Vancomycin and teichoplanin are glycopeptides that are effective against gram-positive bacteria.

Vancomycin isdrug of "last resort",used only after treatment with other antibiotics had failed.

e. Bacitracin (produced by the bacterium Bacillus):

Bacitracin is used topically against gram-positive bacteria.

f. Ethambutol (a synthetic chemical):

Ethambutolis bacteriostatic against actively growing M.tuberculosis bacilli, it works by obstructing the formation of cell wall, That Inhibits incorporation of mycolic acid.


Antimicrobial agents that alter the cytoplasmic membrane.

Alteration of the cytoplasmic membrane of microorganisms results in leakage of cellular materials :

a. Polymyxin B (produced by the bacterium Bacillus):

Polymyxin B is used in severe Pseudomonas infections.

b. Amphotericin B (produced by the bacterium Streptomyces):

Amphotericin B is used for systemic fungal infections.

c. Nystatin(produced by the bacterium Streptomyces):

Nystatin is used mainly for Candida yeast infections.

d. Imidazoles (produced by the bacterium Streptomyces):

The imidazoles are antifungal antibiotics used for yeast infections, dermatophytic infections, and systemic fungal infections. Example Miconazole.


Antimicrobial agents that inhibit protein synthesis

These agents prevent bacteria from synthesizing structural proteins and enzymes.

a. Agents that block transcription (prevent the synthesis of mRNA from DNA):

Rifampins(produced by the bacterium Streptomyces):

Rifampinsare effective against some gram-positive and gram-negative bacteria and Mycobacterium tuberculosis. They inhibit the enzyme RNA polymerase.

b. Agents that block translation (alter bacterial ribosomes to prevent mRNA from being translated into proteins):

1. Agents that bind irreversibly to the 30s ribosomal subunit and cause a misreading of the mRNA (the aminoglycosides produced by the bacterium Streptomyces). Examples include Streptomycin & Amikacin.


2. Agents that bind reversibly to the 30s ribosomal subunit and interfere with the binding of charged tRNA to the bacterial ribosome. Examples include Tetracycline & Doxycycline ( producedby the bacterium Streptomyces). They are effective against a variety of gram-positive and gram-negative bacteria.

3. Agents that bind reversibly to the 50s ribosomal subunit and prevent the release of uncharged tRNA from the bacterial ribosome. Erythromycin is used against gram positive bacteria and some gram-negative bacteria.

c. Antimicrobial agents that interfere with DNA synthesis:

Quinolones (synthetic chemicals): The quinolones block bacterial DNA replication by inhibiting the DNA gyrase, the enzyme needed by bacteria to produce their circular DNA. They are broad spectrum and examples include Norfloxacin, *Nalidixicacid & Ciprofloxacin.

Nalidixic acid : antibiotic used only to treat UTI caused by Gram negative bacteria.

d. Co-trimoxazole (synthetic chemicals):

Co-trimoxazoleis a combination of sulfamethoxazole and trimethoprim,which blocks the bacterial synthesis of folic acid needed to make DNA bases.

microbial resistance to antimicrobial chemotherapeutic agents
Microbial Resistance To Antimicrobial Chemotherapeutic Agents
  • A common problem in antimicrobial chemotherapy is the development of resistant strains of bacteria.
  • A variety of mutations can lead to antibiotic resistance.
  • Most bacteria become resistant to antimicrobial agents by one or more of the following mechanisms:
  • Enzymatic destruction of drug, e.g., penicillinase and other beta-lactamases.
  • Prevention of penetration of drug, e.g., producing an altered cytoplasmic membrane.
  • Alteration of drug's target site, e.g., altered ribosomal subunit.
  • Rapid ejection of the drug.
  • Developing an alternate metabolic pathway.
  • Increasing the production of a certain bacterial enzyme.
  • Resistance genes are often on plasmids or transposons that can be transferred between bacteria.
antimicrobial susceptibility testing by the disk diffusion method kirby bauer antibiotic profile
Antimicrobial susceptibility testing by the disk diffusion method (Kirby-Bauer) & Antibiotic profile
  • Purpose of Procedure
  • To test isolated bacteria for its susceptibility to antimicrobial agents
  • Specimen Requirements
  • In general a pure growth of the isolate.
  • Equipment
  • Sterile cotton swabs.
  • Reagents Required
  • Mueller Hinton Agar plates.
  • Appropriate antibiotic discs or dispenser.
  • McFarland Standard (0.5).
  • Pure culture of the test organism.

The basic steps for the Bauer-Kirby method

of antimicrobial susceptibility testing are:

Preparea standard turbidity inoculum ((0.5) McFarland Standard ) of the test bacterium so that a certain density of bacteria will be put on the plate.

Inoculate a (90 mm diameter, 4 mm agar depth of Mueller-Hinton agar plate) with the standardized inoculum so as to cover the entire agar surface with bacteria.

Placestandardized antibiotic-containing discs on the plate.

Incubate the plate at 37oCfor 24 hours.

Measurethe diameter of any resulting zones of inhibition in millimeters.

Determineif the bacterium is susceptible, intermediate, or resistant to eachantimicrobial agent using a *standardized table.

*Standarizedtable according to manufactory pamphlet.

mueller hinton agar
Mueller Hinton Agar


  • Beef Extract.
  • Digest of Casein ( peptone ).
  • Starch.
  • Agar ....... 1.7 %.
  • Final pH is ~7.3 at 25°C.

Pour cooled Mueller Hinton agar into sterile Petri dishes

on a level, horizontal surface to give a uniform depth of

about 4 mm and cool to room temperature.

0 5 mcfarland standard
0.5 McFarland standard
  • McFarland standards are used as a reference to adjust the turbidity of bacterial suspensions so that the number of bacteria will be within a given range.
  • Were mixing specified amounts of barium chloride and sulfuric acid together.
  • Mixing the two compounds forms a barium sulfate precipitate, which causes turbidity in the solution.
  • A 0.5 McFarland standard is prepared by mixing 0.05 mL of 1.175% barium chloride dihydrate (BaCl2•2H2O), with 9.95 mL of 1% sulfuric acid (H2SO4).
  • Turbidity of 0.5 McFarland standard is approximately equal 1.5X10^8 CFU/mL of bacterial suspension.
  • Adjust the concentration of the inoculum by comparing the turbidity of Normal Saline tube to that of a 0.5 McFarland standard.
  • Note: If the turbidity of the inoculum is greater than that of 0.5 McFarland
  • standard, dilute with sterile normal saline. If the turbidity of the standard is
  • greater than the inoculum, add more of the test organism.
  • 2. Dip a sterile cotton swab into the adjusted inoculum tube and drain excess
  • fluid by pressing the swab against the walls of the test tube.

3. Hold Muller-Hinton plate half or partially open and streak the plate using the

wet cotton swab covering all the area even at the sides.

4. Place the plates aside for about 10 – 15 minutes at room temerature. Allow

the inoculum to dry.


5. Using a sterile forceps or needle, apply a set of suitable antibiotic disks. Five to six disks for each plate, or 8 disks if you use the automatic dispenser.

6.Let the plates stand for 10 - 15 minutes at 4oC, then incubate in inverted position at 37oCfor 18-24 hours.


7. Using a ruler or caliper, measure the zone of inhibition around each

antimicrobial disk and record it.

8. Consult the special chart provided by the manufacturer of the antimicrobial

disks and interpret results as Sensitive, Resistant, or Intermediate.



Any Questions ?