complexity of this problem. For example, Harrison and Lederberg ( 1998 ) defined
antibiotic resistance as a property of bacteria that confers the capacity to inactivate
or exclude antibiotics, or as a mechanism that blocks the inhibitory or killing effects
of antibiotics. Alternatively, the Committee for Veterinary Medicinal Products
(CVMP) in the European Medicine Evaluation Agency (EMEA) (EMEA report:
Antibiotic Resistance in the European Union Associated with Therapeutic Use
of Veterinary Medicines, 1999) defined microbiological resistance as either:
(a) organisms that possess any kind of resistance mechanism or resistance gene;
or (b) an infection where the bacteria do not respond to therapy.
Within a clinical context, antibiotic resistance has traditionally been used to
indicate the likely therapeutic outcome rather than an epidemiological attribute. In
other words, resistance has historically been described in terms of expressed
phenotype. However, more recently, it is being defined in terms of genotype.
This change has lead to difficulties in comparing data generated using different
methodological approaches. For this reason, within the context of an epidemio-
logical study, Davison et al. ( 2000 ) have suggested that the following criteria be
considered:
l Resistance must be regarded as a quantifiable (qualitative or quantitative)
variable at the level of either the bacterial or host population and must be defined
with respect to a reference population.
Table 2(continued)
Class Examples Mechanisms of resistance
Phenicols Chloramphenicol Target site (ribosome) modification/
mutation
Decreased permeability
Inactivating enzymes
Efflux
FlorfencicolPleuromutilins Tiamulin Ribosomal mutations
Ribosomal modification
Decreased Permeability
ValnemulinQuinolones/Fluoroquinolones Nalidixic Acid Target site (DNA gyrase,
topoisomerase IV) mutation
Decreased permeability
Efflux
Target site protection
Ciprofloxacin
Danofloxacin
Difloxacin
Enrofloxacin
Marbofloxacin
Orbifloxacin
Streptogramins Dalfopristin Target site (ribosome) modification/
Pristinamycin mutation
Virginiamycin
Tetracyclines Chlortetracycline Efflux
Ribosomal protection
Drug detoxification
Target site (ribosome) mutation
Doxycycline
Oxytetracycline
Tetracycline
Ionophores Monensin Not known
Narasin
Lasalocid
Antimicrobial Drug Resistance 235