This is a series of lectures on microbiology useful for undergraduate medical and paramedical students

1. ANTI MICROBIAL
CHEMOTHERAPY
Dr. Ashish Jawarkar
M.D.
Consultant Pathologist
Parul Sevashram Hospital

2. Modern era in Antibiotics begins
with Fleming.

3. Definition - Antibiotic





Anti – against
Bio – life
Originally extracted from compounds produced by
living organisms – aminoglycosides
Semi synthetic – cephalosporins, carbapenems
Synthetic – sulphonamides, quinolones,
oxazolidinediones

4. History of Antibiotic Discovery

5. Mechanisms of antibacterial action

6. 
1. Inhibition of cell wall synthesis


2. Inhibition of protein synthesis


Aminoglycosides, tetracyclines, chloramphenicol,
macrolides
3. Alteration of cell membranes


Penicillin, vancomycin, bacitracin
Polymyxin B, Bacitracin
4. Inhibition of nucleic acid synthesis
DNA – quinolones, Metronidazole
 RNA – Rifampicin, Bacitracin


5. Antimetabolite activity

Sulfonamides

7. Antimicrobial Resistance



How bacteria survive doses of antibiotics that
would otherwise have been lethal
Problem – treated with a higher dose, or newer
antibiotic
Side effects, Newer may not be available

9. Types of resistance

Intrinsic


Bacteria may not make the protein that antibiotic
targets
Extrinsic

Due to acquired mutation in DNA or RNA

10. Causes of Resistance


Overuse/misuse of antibiotics
Example is sore throat – Viral , still antibiotics are
prescribed
Failure to take entire course
 Failure to take at correct intervals – two times a day
instead of three times


11. Antibiotic Susceptibility Testing


Pathogenic bacteria show large variations in
susceptibility to antibiotics
Particularly marked in case of Staphylococcus and
gram negative bacteria

12. Types


Diffusion tests
Dilution tests

13. Uses of Antibiotic Sensitivity
Testing



Antibiotic sensitivity test: A laboratory test
which determines how effective antibiotic therapy
is against a bacterial infections.
Antibiotic sensitivity testing will control the use of
Antibiotics in clinical practice
Testing will assist the clinicians in the choice of
drugs for the treatment of infections.

14. Components of Antibiotic
Sensitivity Testing


1.The identification of relevant pathogens in
exudates and body fluids collected from patients
2. Sensitivity tests done to determine the degree of
sensitivity or resistance of pathogens isolated from
patient to an appropriate range of antimicrobial
drugs

15. Antibiotic Sensitivity Testing Is
Essential of selection of
Antibiotics

16. Isolation and Identification of Bacteria
precedes the selection of Antibiotic
Testing Methods

17. Uses of Antibiotic Sensitivity
Testing



Helps to guide the Physician in choosing
Antibiotics
The accumulated results on different pathogens
their sensitivity will guide the physician in
choosing empirical treatment in serious patients
before the individual’s laboratory results are
analyzed in the Microbiology laboratory.
Reveals the changing trends in the local isolates.

18. Why Need continues for testing for
Antibiotic Sensitivity

Bacteria have the ability to
develop resistance following
repeated or subclinical
(insufficient) doses, so more
advanced antibiotics and
synthetic antimicrobials are
continually required to
overcome them.

19. Testing for Antibiotic sensitivity



The method includes several steps including
obtaining a bacterial sample;
identifying the type of bacteria in the bacterial
sample; selecting a set of antibiotics based on the
identity of the bacteria in the bacterial sample;
obtaining a control sample from the bacterial
sample;

20. What is Resistance in Antibiotic
Sensitivity Testing

Resistance implies that the infection is not
treatable with the tested Antibiotic because its
MIC exceeds achievable safe tissue or urine
levels.

21. Kirby-Bauer methods
A commonly used method in basic
laboratories

Kirby-Bauer antibiotic testing (KB testing or
disk diffusion antibiotic sensitivity testing) is a
test which uses antibiotic-impregnated wafers to
test whether particular bacteria are susceptible to
specific antibiotics

22. How to perform Kirby- Bauer
testing

The basics are easy: The bacterium is swabbed on
the agar and the antibiotic discs are placed on top.
The antibiotic diffuses from the disc into the agar
in decreasing amounts the further it is away from
the disc. If the organism is killed or inhibited by
the concentration of the antibiotic, there will be
NO growth in the immediate area around the disc:
This is called the zone of inhibition .

23. Steps in Antibiotic sensitivity testing

24. Bacteria are inoculated as
lawn culture

Method of inoculationGood results are
obtained by placing a
standard loopful of
inoculum suspension on
the plate and then
spreading it with a dry
sterile swab.

25. The area of Inhibition is measured
with a Scale
 Record
the
results for
everyone on
your table in the
table below.

26. The disk diffusion methods are
commonly used for routine testing

27. Read the plates in
transmitted light

28. The zone of inhibition guides
the right choice of Antibiotic

29. The strips with multiple
Antibiotics can be tested in one go

30. Other methods of Antibiotic
susceptibility testing

Other methods to test antimicrobial susceptibility
include the Stokes method, E-test (also based on
antibiotic diffusion). Agar and Broth dilution
methods for Minimum Inhibitory Concentration
determination.

31. Testing Minimum Inhibitory
Concentration

In alternative measure of susceptibility is to
determine the Minimum Inhibitory
Concentration (MIC) and the Minimum
Bactericidal Concentration (MBC) of a drug. A
series of broths are mixed with serially diluted
antibiotic solutions and a standard inoculum is
applied. After incubation, the MIC is the first
broth in which growth of the organism has been

32. What is Minimum Inhibitory
concentration

Minimum inhibitory concentration (MIC), in
microbiology, is the lowest concentration of an
antimicrobial that will inhibit the visible growth of
a micro organism after overnight incubation.
Minimum inhibitory concentrations are important
in diagnostic laboratories to confirm resistance of
micro organisms to an antimicrobial agent and
also to monitor the activity of new antimicrobial
agents.[

33. The Antibiotics are diluted to various
dilution to test the minimum
inhibitory concentration

34. What is E Test

Etest is an antimicrobial gradient technique in
which 15 reference MIC dilutions of an antibiotic
have been repackaged with innovative dry
chemistry technology onto a plastic strip. The
predefined gradient provides precise and accurate
assessment of antimicrobial activity against both
fastidious and non-fastidious microorganisms.

Dr.T.V.Rao MD

35. The s t ri ps are
im
pregnat ed w t h vari ous
i
c onc ent rat i on of
Ant i bi ot i c s

36. E = testing on various
isolates

37. Limitation of Disk Diffusion
Methods


Disk diffusion methods are
not suitable for slow
growing bacterial pathogens
The great limitation being
for testing on
Mycobacterium
tuberculosis which needs
varied, technically
demanding methods

38. Antibiotic Sensitivity testing can
be done with automation

39. There is a growing need for
Automation in Antibiotic sensitivity
testing

40. Limitation of Antibiotic Sensitivity
usage
Both Microbiologists and Clinician should however
bear in mind that the response therapy in vivo may
not always reflect the results of testing the
sensitivity of patient's pathogen in vitro.

41. Thank You