Chemotherapeutic
and Chemical Agents
Microbiology 156
Antibiotics
Drugs that interfere with bacterial cell
metabolism
 They produce a bacteriocidal or
bacteriostatic effect on prokaryote
cells

Origin of Antibiotics
These are agents that have been
synthesized by bacteria or fungi against
other organisms
 Example – Penicillin was isolated from
the mold Penicillium notatum. It was
first observed by Alexander Fleming
and later purified by Howard Florey.

Antibiotics inhibit the growth of
other microorganisms
Mode of Action
Interfere with the synthesis of the cell
wall( peptidoglycans)
 Disruption of the cell membrane
 Inhibition of folic acid
 Interference with protein synthesis
 Interference with nucleic acid synthesis
 Ribosomal interference

Two Pioneers in
Chemotherapeutic agents
Penicillin – The first


Effects the
synthesis of the
peptidoglycans cell
wall
Makes the bacteria
vulnerable to the
phagocytes in the
immune system
Chemical structure of antibiotics
based upon the original penicillin
Antibiotics from Actinomycetes



Selman Waksman,
Rutgers University, NJ
Actinomycetes are
soil bacteria
They produce
compounds that
inhibit other
microorganisms
Competition for food
and space in the soil
environment
Streptomycin
Inhibition of bacterial growth by
soil bacteria
Pharmacology
Synthetic drugs are designed based
upon the structure of molecules
produced in nature by bacteria
 They are then tested for their
antimicrobial activity – in vitro and in
vivo

KirbyBauer
The determination of antibiotic
sensitivity or resistance
 Sensitivity – The organisms growth is
inhibited by the presence of the
antibiotic
 Resistance – The organism has a means
of interfering with the mode of action
of the antibiotic

Kirby Bauer
Plate bacteria on Mueller Hinton Agar
Plate
 Distribute bacteria evenly over the
surface of the agar
 Place discs impregnated with antibiotic
on the agar at indicated spots.
 Incubate

Determination of antibiotic
resistance or sensitivity
Chemical agents
Incubate the plates for 24 hours
 Measure the diameter of the plate to
determine efficacy of the chemical
agents

Determination of resistance or
sensitivity
Measure the diameter of the ring
around the disc
 Compare the measurement in mm with
the table in your lab manual, page 284.
Table 44.2
 Record your results in Table on page
287.

Control of Microbial GrowthGrowth Curve
Heat- Destruction of bacterial
cells
Autoclave – heat under pressure
 121 0C at 15 psi for 15 minutes
Boiling
 For more than 30 minutes
Chemical agents
Agent
Mechanism of
Action
Use
Peroxygens
Oxidations
Use in wounds and
abrasions
Alcohols
Protein denaturation
Skin disinfection
Organic acids
Disrupt proteins- low Used to control
pH
moleds
Detergents
Disrupts cell
membranes
Disinfection
Halogens derivative
Iodine
Chlorine
Inhibits protein
action in bacteria
Surface area
disinfection
Chemical Agents- Agar Plate
sensitivity assay
Plate the bacteria by spreading it evenly
across the Mueller- Hinton agar.
 Dip sterile filter discs into
chemotherapeutic agents.
 Carefully place the discs in indicated
spots on the agar plate

Chemical agents
Torr solution
 Chlorox solution
 Mouth wash
 Lysol solution
 Other

Determination of activity

Measure the zone of inhibition around
the disc to determine if the antiseptic
or disinfectant had sufficient activity
to inhibit the growth of the bacteria in
your study
Chemical Agent Activity Assay
Organism
E. coli
SA
Other
Antiseptic
Disinfectant