Antimicrobial Drug Sensitivity
testing and therapeutic use in
Section of Epidemiology,
CADRAD, IVRI, Izatnagar-243122,
• The term antibiotic was coined by Selman
Waksman in 1942 to describe any
substance produced by a microorganism
that is antagonistic to the growth of other
microorganisms in high dilution.
• The term antibiotic, meaning “substances
Rules of Antibiotic use
(Marino, 2007, The ICU Book)
• Try not to use them,
• Try not to use too many of them.
• (1) Why do we test antimicrobial
• (2) How do we perform antimicrobial
• (3) How can we detect resistance
• (4) Why & how do we assay antimicrobial
Deciding on whether to use an
• Clinical Practice:
– Day 1: guess the disease, guess the bug, guess the
sensitivity pattern and administer the treatment.
• Standard Practice:
– Day 1 Clinical assessment
– Day 2 Positive microbiology
– Day 3 Antibiotic sensitivity tests
– BUT TOO LATE?
• Other factors are very important when we
choose an antibiotic
• The history of previous antimicrobial use.
• Will it get to where the infection is?
• Likelihood of development of resistance.
Why do we test antimicrobial
• To direct & predict antimicrobial
• To review & monitor epidemiological trends.
• To set national & local antibiotic policies.
• To test the activity of a new antimicrobial
• To presumptively identify isolates.
How do we perform antimicrobial
• We can use a number of methods including:-
• Disc susceptibility tests - Kirby-Bauer
• Agar Breakpoint method.
• Minimum Inhibitory Concentration (MIC) – Tube
MIC or E-tests.
• Automated methods – Vitek.
• Molecular methods – PCR.
Disc Susceptibility Tests
• Developed in the USA in 1966.
• Based on NCCLS ( National
Committee for Clinical Laboratory
Standards ) data.
• Use Mueller-Hinton agar.
• Use standard 0.5 McFarland
• Streak inoculum in 3 directions or
• Use standard NCCLS tables to
interpret zone sizes as S, I or R.
Stokes’ Comparative Method
• Developed in the U.K (1972).
• A variety of media can be used including Iso-
sensitest agar (ISA), ISA & 5% horse blood &
• Use NCTC (National Collection of Type
Cultures) controls e.g. NCTC 6571 Staph
aureus, NCTC 10602 Ps. aeruginosa, NCTC
• Using a rotary plater apply the control
suspension on the outer edge & the test
suspension in the centre, leaving a gap for the
• Interpretation based on comparison between
zones seen with the test organism & those of
the known sensitive control.
• Sensitive = zone radius of test, larger, equal, or
not more than 3mm smaller than the control.
• Intermediate = zone radius more than 3mm, but
smaller than the control by more than 3mm.
• Resistant = zone radius of 3mm or less.
• Developed in the U.K in 1998 by BSAC
working party, through statistical
regression analysis of zone diameter &
MIC data, on hundreds of strains.
• Use ISA and/or ISA & 5% horse blood &
• Use standard 0.5 McFarland inoculum
• Use dilution protocol of 0.5 McFarland
standard as specified by BSAC.
• Neat 0.5 Neisseria gonorrhoeae.
• 1:10 dilution of 0.5 Staphylococci.
• 1:10 dilution of 0.5 for Streptococci.
• 1:100 dilution of 0.5 for Coliforms.
• 1:100 dilution of 0.5 for Haemophilii
• Set up a series of antibiotic doubling dilutions in tubes containing liquid
media (ISA or ISA with lysed blood).
• For example 128mg/l, 64, 32, 16, 8, 4, 2, 1, 0.5, 0.25, 0.125, 0.
• Set up tubes for test organism & NCTC control organism.
• Add standard organism inoculum to each tube.
• Include an antibiotic free tube i.e organism only.
• Include an organism free tube i.e antibiotic only.
• Incubate tubes overnight.
• Examine for presence of growth by shaking each tube & observing
• Check antibiotic free tube has growth.
• Check organism free tube has no growth.
• Check the NCTC control gives the recommended MIC.
• The MIC is the first tube dilution without visible growth.
• The tube MIC is very labour intensive, difficult to get right & prone to error.
E-tests [AB Biodisk,Sweden]
• A commercial alternative to tube MIC.
• Consists of a plastic strip 6cm by 0.5cm
• Exponential gradient of antimicrobial
dried on one side.
• MIC scale printed on the other side.
• The range corresponds to fifteen 2-fold
• Follow manufacturers’ instructions for
inoculum preparation, media
recommendations & incubation
• MIC interpretation made where growth of
inhibition ellipses the strip.
• Most E-test require examination with a
hand-lens to look for minute colonies
intersecting the strip.
• Three main methods are in use in the U.K.
• These include the Vitek (Biomerieux), the Phoenix
(Becton-Dickinson) & the Mastascan Elite (Mast).
• Not in clinical use in India.
• The Vitek I was originally designed by NASA for use as
an on-board space exploration test system.
• It is based on the use of small thin plastic cards each
containing many wells linked by capillaries.
• These cards are available as susceptibility &
Vitek advantages / disadvantages
• Can give rapid “expert” results for
identification & susceptibility.
• It can be directly linked to the LIMS.
• It decreases the incidence of operator
error & can be operated by an MLA.
• It is very expensive, approx £7 for an id &
• It needs regular software updates & expert
analysis by a BMS.
Why & how do we assay
antimicrobial serum levels?
• We assay to ensure
adequate therapeutic levels &
to avoid the accumulation of
• Most antimicrobial agents
have a large therapeutic
index & are given in large
doses without causing harm.
• The aminoglycosides, the
glycopeptides & some
antifungals have a narrow
therapeutic range which can
be close to the toxic range.
Antibiotics contraindicated in Horses
Erythromycin (in adults),
Neomycin, gentamicin in foals (by injection also),
Amphotericin B (should not be used locally)
Antibiotics contraindicated in Dogs and Cats
Tobramycin, spectinomycin, Gentamicin (neuromuscular &
renal toxicity), phenylbutazone, chloramphenicol and
griseofulvin, cefachlor/ cefadroxil/ cephalexin/ cepharadine
(cause diarrhoea and vomiting), enerofloxacin/
ciprofloxacin/ norfloxacin (may induce iarrhoea and
vomiting, nephrotocity, must be avoided in pregnant and
neonates), erythromycin/ clindamycin/ lincomycin (cuases
vomiting, diarrhoea and hepatotoxicity), metronidazole
(avoid in early pregnancy, anorexia, vomiting, neuro
problems), nitrofurans and rifampin (hepatotoxic, discolour
tears and urine, avoid during pregnancy), ketoconazole,
itraconazole (anorexia, depression, dirrhoea, vomiting, coat
change, in cats fever, anemia)
Antimicrobial contraindicated in
Antibiotics should not be used orally and those
antibiotics have hepatobiliary excretion should be
avoided by injections too.
Tetracyclines, sulfonamides and coccidiostats can
be used orally. Monensin, a coccidiostat, has often
problem with uniform mixing in feed thus poisoning
Swines Sulfamethazine (due to residue in meat), monensin,
salinomycin, arsanilic acid (cuases muscle tremors,
Furazolidones, Nitrofurans, Apramycin,
dimetridazole, Metronidazole (Not approved for
Penicillins, tetracyclines, clindamycin or lincomycin
causes Enteritis, profuse diarrhoea and death in 4-5
days, enterotoxemia due to Clostridial-infection. In
rabbits diarrhoea due to Clostridium spiroformae is
common after antibiotic use.
Dihydro-streptomycin (Cardiovascular effect death
Pet birds Tetracyclines during egg production and breeding
season results in to soft shelled eggs and rickets in
Disease/ Pathogen Drugs of choice
Brucellosis Tetracyclines, chloramphenicol, gentamicin
Edwardsiellosis Azithromycin, aztreonam, ceftriaxone, fluoroquinolones,
Escherichia coli Nitrofurantoin, gentamicin, ceftriaxone+tazatobactam,
Klebsiella pneumoniae Fluoroquinolones, gentamicin, imipenem, meropenem,
Moraxella bovis Strepto-penicillin, and most of the common antibiotics
Pasturellosis Tetracycline, fluoroquinolones, amoxycillin, gentamicin,
Salmonellosis Chloramphenicol, gentamicin,
ceftriaxone+tazatobactam, cotrimoxazole, imipenem
Staphylococcal infections Tetracycline, nitrofurantoin, chloramphenicol,
Streptococcal infections Clindamycin, gentamicin, amoxyclav, vancomycin,
Pseudomonas infections No, use only after testing
Guidelines to good antimicrobial stewardship
1. Arm yourselves with a working knowledge of commonly used antimicrobials.
2. If possible, a narrow spectrum should be chosen
3. Prophylactic use should generally be avoided except in case of surgery.
4. If the use is justified, the correct dose, dose frequency and duration of treatment
should be used.
5. Topical or local use should always be considered.
6. Use only rational combinations of antimicrobials if considered necessary.
6. Routine use of fluoroquinolones, third- and fourth-generation cephalosporins and
amikacin should be absolutely avoided. Similarly , ‘last resort’ antimicrobials, such as
imipenem and vancomycin, should not be used for veterinary patients.
7. Take time to while prescribing antimicrobials.
8. Create a table of first-, second- and third-choice antimicrobials for your practice.
First-choice antimicrobials would comprise agents appropriate for initial treatment,
not necessarily based on culture and sensitivity information.
Second-choice- prescribed based on culture and sensitivity data,
Third-choice antimicrobials should only be prescribed for serious and life-
threatening infections, based on culture and sensitivity data, and only where no first-
or second-choice agents are appropriate.
9. Keep an eye on guidelines reviewed regularly about antimicrobial drug use.
10. Report treatment failures. This is a vital part of monitoring for developing and