Page 263 - The Veterinary Laboratory and Field Manual 3rd Edition

232 Susan C. Cork and Roy Halliwell

Table 4.4b Mode of Action of some commonly used antibacterial drugs.

Antibacterial drug Mode of Action Comments
BACTERIOCIDAL
ß – Lactam antibiotics Inhibition of cell wall Low toxicity
e.g. Penicillins, Cephalosporins synthesis
Aminoglycosides, e.g. Inhibition of protein May be ototoxic and nephrotoxic
Steptomycin, Neomycin synthesis
Trimethoprim Inhibition of nucleic acid Usually administered with
synthesis sulphamethaxazole as a ‘potentiated
sulphonamide
Vancomycin Inhibition of cell wall May be used in cases of methicillin
synthesis resistant Staphylococcus aureus (MRSA)
Polypeptides, e.g. Polymixin Inhibition of membrane May be nephrotoxic and neurotoxic
function
BACTERIOSTATIC
Nitrofurans Inhibition of protein Relatively toxic. Broad spectrum of
synthesis activity
Tetracyclines Inhibition of protein Development of resistance common,
synthesis has been widely used for prophylactic
treatment in livestock
Chloramphenicol Inhibition of protein Potentially toxic. Use prohibited in food
synthesis producing animals in many countries
Macrolides, e.g. erythromycin, Inhibition of protein May be effective against Mycoplasma sp.
Tylosin synthesis
Quinilones, e.g. Enrofoxican, Inhibition of nucleic acid May be used in combination with other
Nalidixic acid synthesis drugs for treatment of mastitis
Sulphonamides Inhibition of nucleic acid Effective against rapidly growing bacteria
synthesis
Nitroimidazoles, e.g. Disruption of DNA structure Effective against anaerobic bacteria and
Metronidazole and inhibition of repair some protozoa, e.g. Giardia

spontaneous mutations in the loci on the micro- MuLtIPLE rESIStancE
bial chromosome that determine susceptibility The development of multiple resistance in bacteria
to specific antibiotics. It is thought that the depends on several different mechanisms. More
presence of the antibiotic serves as a selecting than one mechanism may operate for the same
mechanism to suppress susceptible microor- antibiotic. Microorganisms resistant to a certain
ganisms and promote the growth of resistant antibiotic (see Table 4.4b) may also be resistant to
strains. Spontaneous mutations can also occur other antibiotics that share a mechanism of action
and these are transmissible vertically, that is, or attachment. This type of ‘cross-resistance’ is
they are passed on during bacterial replication. most common with antibiotics that are closely
related chemically (for example, polymyxin B and
colistin, neomycin and kanamycin), but may also

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