Page 259 - The Veterinary Laboratory and Field Manual 3rd Edition

228 Susan C. Cork and Roy Halliwell

a cost-saving measure. For example: prepare a be more efficient to only assay antibiotics that
solution of ampicillin with final concentration are available for subsequent treatment.
of 0.5 mg/ml (0.5 µg/µl), add 20 µl to each disc,
and let dry. This will result in 10 µg of ampicil- Examples of suitable ‘first-line’ test
lin per disc. antibiotics
Dry discs can be stored at –20º C for at least
a year without loss of potency and for a shorter 1 For cultures from specimens other than urine:
period at 4ºC. The manufacturer’s expiry date (for example, blood, exudates, pus, sputum,
should be observed for purchased discs, and swabs from wounds and so on) test ampicil-
the antibiotic expiry date must be observed for lin, cotrimoxazole, erythromycin, penicillin
in-house prepared discs. Wet discs are also com- (benzyl), tetracycline and gentamicin (or
mercially available, these retain their potency for cephaloridine or streptomycin).
several months at 4ºC. 2 For cultures from urine test ampicillin, cotri-
Figure 4.13a outlines the wide range of fac- moxazole, nalidixic acid, nitrofurantoin,
tors that should be taken into consideration sulfonamide and kanamycin (or cephalori-
before an antibiotic is prescribed for an animal. dine or tetracycline).
It can be seen that determining the antibiotic 3 For Staphylococcus spp. test erythromycin,
sensitivity of bacteria isolated from a clini- fucidin, penicillin (benzyl), tetracycline, lin-
cal sample will assist the veterinary or other comycin (or clindamycin), cotrimoxazole (or
animal health professional to make a decision gentamicin or movabiocin).
with regard to treatment, but that other factors 4 For other Gram +ve cocci test ampicillin,
may be important. In some cases, a bacterial cephaloridine cotrimoxazole, erythromycin,
isolate which shows limited sensitivity to an penicillin (benzyl).
antibiotic in the laboratory may still be effec- 5 For most Gram –ve bacteria test ampicillin,
tive when given to the animal. Generally, only cephaloridine, cotrimoxazole (or sulphon-
qualified veterinary professionals are permitted amide), streptomycin (or kanamycin) and
to prescribe medicines and to treat animals tetracycline, for Salmonella typhi include chlor-
so recommendations should be made follow- amphenicol.
ing discussion with the veterinary officer in 6 For Haemophilus sp. test ampicillin, cepha-
charge. loridine, chloramphenicol, cotrimoxazole,
sulfonamide and tetracycline.
7 For Pseudomonas aeruginosa test carbenicillin,
Choice of drug for disc test
colistin (or polymyxin B) and gentamicin.
When large numbers of sensitivity tests have to 8 For Clostridia and other anaerobic Gram +ve
be done on isolates from clinical specimens, it bacilli test ampicillin, clindamycin, fucidin,
is convenient to restrict the routine ‘first-line’ penicillin (benzyl) and tetracycline.
tests to the number of antibiotics that can be
accommodated on a single culture place. It is Culture medium
standard practice to put six to eight antibiotic
discs on a 140 mm plate. Using more than this A nutrient medium should be used which is as
may cause the zones of inhibition to overlap. In free as possible from substances inhibitory to the
special cases requiring tests against additional action of the antibiotics (and sulphonamides).
antibiotics, a second plate may be set up with a Muller-Hinton agar is the most commonly used
supplementary set of discs. In some cases, it may growth medium for disc diffusion assays. The

Vet Lab.indb 228 26/03/2019 10:25