Page 520 - ONLINE PROCEEDING BOOK WSAVA 2017
P. 520

An Urban Experience
WSVA7-0535
INFECTIOUS DISEASES I
PRATICAL GUIDE TO INTERPRETATION OF ANTIMICROBIAL SUSCEPTIBILITY TEST
J. Sykes1
1University of California- Davis, Department of Medicine & Epidemiology, Davis, USA
PRACTICAL GUIDE TO INTERPRETATION OF SUSCEPTIBILITY TEST RESULTS
In recent years, there has been a frightening rise in the prevalence of multidrug-resistant bacterial infections
in dogs and cats. Because of this, whenever possible, veterinarians should make attempts to con rm a suspected bacterial infection by requesting microscopic evaluation of direct smears, culture and susceptibility
by a laboratory before the choice is made to administer an antimicrobial drug. Cytology can permit the rapid preliminary diagnosis of infection. Whether organisms seen are cocci or rods helps guide the clinician to select an appropriate empiric therapy, if necessary, while awaiting the results of culture and susceptibility testing. Important terms used to describe resistant bacterial infections are as follows:
Beta lactam: antimicrobial drug that includes a beta-lactam ring (all penicillins, cephalosporins and carbapenems such as meropenem). These bind to penicillin binding proteins (PBPs) (bacterial enzymes that catalyze bacterial cell wall formation) and cause bacterial lysis.
Beta lactamases: bacterial enzymes that destroy the beta lactam ring (associated with resistance to beta-lactams). These include a variety of penicillinases. Beta lactamase inhibitors are drugs that inhibit these enzymes and include clavulanic acid and sulbactam.
ESBLs: extended spectrum beta lactamases. These are bacterial enzymes that destroy critical beta-lactam drugs needed for treatment of resistant bacterial infections in humans (by de nition, third generation cephalosporins such as cefuroxime, cefotaxime, ceftazidime). They are generally expressed by gram-negative enteric bacteria such as E. coli and Klebsiella.
MRS: methicillin resistant staphylococcus. These organisms express an altered penicillin binding protein (PBP2a) that does not bind beta-lactam drugs. Therefore they are resistant to penicillins, cephalosporins and carpapenems.
MDR: multidrug resistance. By de nition, this is resistance to 3 or more CLASSES of antimicrobial drugs (e.g., cephalosporins,  uoroquinolones, and aminoglycosides).
METHODS OF SUSCEPTIBILITY TESTING
Clinical microbiology laboratories will perform susceptibility testing for most aerobic bacteria, with the exception of streptococci. Streptococci from dogs and cats are almost always susceptible to penicillins. Most laboratories also do not routinely perform susceptibility testing on anaerobes, which also mostly have predictable susceptibilities, although resistance in anaerobes is increasing and some anaerobes, such as Bacteroides fragilis, have a high prevalence of β-lactamase enzyme production.
Susceptibility testing can be performed using dilution methods or diffusion methods. The minimum inhibitory concentration (MIC) is the lowest concentration of antimicrobial drug that inhibits visible growth of an organism over a de ned incubation period, most commonly 18 to 24 hours, and is determined using dilution methods, which involve exposing the organism to 2-fold dilutions of an antimicrobial drug. The concentration range used varies with the drug and the organism being tested. Standard protocols are published by the Clinical and Laboratory Standards Institute (CLSI) that specify medium composition and pH, inoculum size (determined on the basis of turbidity measurements), inoculation procedures, agar depth and incubation conditions, as well as quality control requirements. Because failure to comply with these protocols can
lead to erroneous results, veterinarians should always attempt to use laboratories that follow CLSI or EUCAST protocols.
The most widely used dilution method is broth microdilution, whereby 2-fold dilutions of antimicrobials are made in a broth media in a microtiter plate. Pre- prepared frozen or freeze-dried plates are available commercially for inoculation (e.g., SensititreÒ plates, TREK Diagnostic Systems). The results can be determined using visual examination of the plates for
the inhibition of bacterial growth, or by the use of semi- automated or automated instrumentation. The MIC for each antimicrobial drug tested against the organism is reported to the clinician on the susceptibility panel. It is the lowest concentration of antibiotic (usually in mg/mL) that inhibits growth of the organism in vitro, and the lower the MIC, the more potent the antimicrobial is at inhibiting bacterial growth.
Diffusion methods include gradient diffusion (also known as Etest®) and disk diffusion. The Etest involves use
of a plastic strip coated with an antimicrobial gradient on one side and an MIC interpretive scale on the other side. An agar plate is inoculated with the organism of interest so that subsequent growth of the organism will form a “lawn”, rather than individual colonies. The strips
520
42ND WORLD SMALL ANIMAL VETERINARY ASSOCIATION CONGRESS AND FECAVA 23RD EUROCONGRESS


































































































   518   519   520   521   522