Page 262 - The Veterinary Laboratory and Field Manual 3rd Edition

Microbiology 231

a notE on antIMIcrobIaL rESIStancE and animals and the development of resistance
Over the past few decades a wide range of highly in environmental, commensal and some poten-
effective antimicrobial drugs have been devel- tially pathogenic bacteria. The degree to which
oped to control bacterial diseases in humans the use of antibiotics in veterinary medicine con-
and animals. Much of this success has been due tributes to antimicrobial resistance in bacteria
to our greater understanding of how bacteria isolated from humans is not currently known
cause disease (Table 4.4a) and the development but it is evident that improper use of antibiotics
of antimicrobial drugs targeted against specific in agriculture and aquaculture can lead to resis-
bacterial virulence factors. Many of the newer tance in bacteria isolated from farmed livestock
classes of drugs target a narrow range of bacteria and fish. See also Chapter 13.
thereby reducing the side effects associated with
depletion of normal gut flora. However, antibi- acquIrEd rESIStancE
otic use, especially when antibiotics are not used Antibiotic resistance can occur when genetic
correctly (that is, when used to enhance produc- material is transferred between bacteria, this
tion in healthy animals or where underdosing of can occur in several ways; for example, via plas-
clinical cases occurs), may lead to the selection mids, which are small extrachromosal DNA
of resistant forms of microorganisms. A variety molecules or via integrons and transposons,
of antimicrobial resistance mechanisms have which are short DNA sequences, can be trans-
been identified in bacteria and some of these are mitted both vertically (that is, from bacteria to
outlined in Table 4.4b. Microorganisms are con- bacteria) and horizontally (during replication)
stantly evolving. It is known that resistance to and can code for multi-resistance. It is thought
antibiotics existed long before antibiotics were that a lot of the acquired resistance observed in
widely used. However, there is now a demon- bacteria is plasmid-mediated. In comparison,
strated link between antimicrobial use in humans mutational resistance develops as a result of

Table 4.4a Bacterial mechanisms to enhance survival in the host.

Mechanism Example
Capsule production Many bacteria use this to prevent or limit phagocytosis
Capsular antigen Some Gram –ve bacteria incorporate sialic acid which has a
–ve effect on complement fixation
M protein production Streptococcus equi uses this to prevent phagocytosis
Antigenic variation of surface antigens Facilitates evasion of the host immune response, seen with
Mycoplasma sp. and Borrelia sp. infections
Antigenic mimicry of host antigens Facilitates evasion of the host immune response, seen with
Mycoplasma sp. (also seen with some parasitic infections)
Production of leukotoxins Mannheimia haemolytica, Actinobacillus sp. and others
produce toxins that can lyse phagocytes
Escape from phagosomes Listeria monocytogenes and rickettsiae
Interference with phagosome-lysosome Mycobacteria use this to survive within phagocytes
fusion
Coagulase production Staphylococcus aureus may convert fibrinogen to fibrin to
isolate the site of infection from the active immune response

Vet Lab.indb 231 26/03/2019 10:25