Antibiotic Lab 10

Topics: Bacteria, Microbiology, Antibiotic resistance Pages: 8 (2300 words) Published: October 4, 2014
LabPaq: MBK

Page 129

Antibiotic Sensitivity
Cynthia Alonzo, M.S. Version 09-2.01
Review the safety materials and wear goggles when working with chemicals. Read the entire exercise before you begin. Take time to organize the materials you will need and set aside a safe work space in which to complete the exercise.

 To understand the basic principles of antimicrobial therapy;  To become familiar with the phenomenon of antibiotic resistance; and  To become familiar with and employ an antibiotic sensitivity test. Materials From:

Label or Box/Bag:

Student Provides

From LabPaq

Auxiliary Items Bag

Auxiliary Items BagMBK



Distilled water
10%-bleach Solution
30%-bleach Solution
Culture of S. epidermidis
Gloves packages - 11 pairs
Ruler, Metric
Tweezers, plastic
Antibiotic Disk - Gentamicin in Bag 2"x
Antibiotic Disk - Novobiacin in Bag 2"x
Antibiotic Disk - Penicillin in Bag 2"x 3"
Sterile Swabs - 2 per Pack


Mask with Ear loops (11) in Bag 5" x 8"


Inoculation Instruments
Mask Bag

Mask Bag

Item Description:

Pre-Lab Preparation: Place saved culture of S. epidermidis (from previous lab) in incubator 12-24 hours prior to the start of the experiment.
Discussion and Review: Antimicrobial therapy is the use of chemicals to inhibit or kill microorganisms in or on the host. Drug therapy is based on selective toxicity. This means that the agent used must inhibit or kill the microorganism in question without seriously harming the host.

In order to be selectively toxic, a therapeutic agent must interact with some microbial function or microbial structure that is either not present or is substantially different from that of the host. For example, in treating infections caused by prokaryotic bacteria, the agent may inhibit peptidoglycan synthesis or alter bacterial (prokaryotic) ribosomes. Human cells do not contain peptidoglycan and possess eukaryotic ribosomes. Therefore, the drug shows little if any effect on the host (selective toxicity). Eukaryotic microorganisms, on the other hand, have structures and functions more closely related to those of the host. As a result, the variety of agents selectively effective against eukaryotic microorganisms such as fungi and protozoans is small when compared to the number available against prokaryotes. Also keep in mind that viruses are not cells Hands-On Labs, Inc.

LabPaq: MBK

Page 130

and, therefore, lack the structures and functions altered by antibiotics so antibiotics are not effective against viruses.
Based on their origin, there are 2 general classes of antimicrobial agents: 

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

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



Today the distinction between the 2 classes is not as clear, since many antibiotics are extensively modified in the laboratory (semisynthetic) or even synthesized without the help of microorganisms.

Most of the major groups of antibiotics were discovered prior to 1955, and most antibiotic advances since then have come about by modifying the older forms. In fact, only 3 major groups of microorganisms have yielded useful antibiotics: the actinomycetes (filamentous, branching soil bacteria such as Streptomyces), bacteria of the genus Bacillus, and the saprophytic molds Penicillium and Cephalosporium. To produce antibiotics, manufacturers inoculate large quantities of medium with carefully selected strains of the appropriate species of antibiotic-producing microorganism. After incubation, the drug is extracted from the medium and purified. Its activity is standardized and it is put into a form suitable for administration. Some antimicrobial agents are cidal in action: they kill microorganisms (e.g., penicillins,...
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