Antibiotic therapy has made many advances that has given veterinary medicine a large number of effective drugs and provided pharmacokinetic and pharmacodynamic information to guide dosing. New approaches to bacterial identification and susceptibility testing has helped to provide information for the most appropriate drug selection. This paper will review the current concepts that guide antibiotic therapy in veterinary medicine and provide important strategies for effective dosing.
If the bacteria is accurately identified, antibiotic selection is simplified because the susceptibility pattern of many organisms is predictable. For example, if the bacteria is likely to be Pasteurella, Streptococcus, or Actinomyces, susceptibility is expected to penicillin or an aminopenicillin such as ampicillin, amoxicillin, or amoxicillin-clavulanic acid (Clavamox).
Usually Susceptible Bacteria
Staphylococcus isolated from small animals is most likely to be S. intermedius rather than S. aureus. S. intermedius will usually have a predictable susceptibility to ß-lactamase resistant ß-lactam antibiotics such as amoxicillin combined with a ß-lactamase inhibitor (Clavamox), or a first-generation cephalosporin such as cephalexin or cefadroxil. Staphylococcus also is susceptible to oxacillin and dicloxacillin but these are not used as commonly in small animal medicine. Reports of studies on S. intermedius have shown that, despite frequent use of the above-mentioned drugs in small animals, the incidence of resistance has not increased (Lloyd, et al., 1996). Most staphylococci are also sensitive to fluoroquinolones. The majority of staphylococci are sensitive to lincosamides (clindamycin, lincomycin), trimethoprim-sulfonamides, or erythromycin, but resistance can occur in as high as 25% of the cases.
If the bacteria is an anaerobe (for example, Clostridium, Fusobacterium, Prevotella, Actinomyces, or Porphyromonas) predictable results can be attained by administering penicillin, chloramphenicol, metronidazole, clindamycin, amoxicillin-clavulanic acid, or one of the second-generation cephalosporins such as cefotetan or cefoxitin. Metronidazole is consistently highly active against anaerobes including B. fragilis. The activity of first-generation cephalosporins, trimethoprim-sulfonamides/ormetoprim-sulfonamides, or fluoroquinolones for an anaerobic infection is unpredictable. If the anaerobe is from the Bacteroides fragilis group, resistance may be more of a problem because they produce a beta-lactamase that may inactivate 1st generation cephalosporins and ampicillin/amoxicillin. Some of these Bacteroides may also be resistant to clindamycin. These resistant strains of Bacteroides may have increased in recent years (Jang et al., 1997).
Problem, or Resistant Bacteria
If the organism is Pseudomonas aeruginosa, Enterobacter, Klebsiella, Escherichia coli, or Proteus, resistance to many common antibiotics is possible and a susceptibility test is advised. For example, a recent report showed that among nonenteric E. coli, only 23% were sensitive to a 1st generation cephalosporin, and less than half were sensitive to ampicillin. In that same study, 13%, and 23% were intermediate or resistant to enrofloxacin, and orbifloxacin, respectively (Oluoch, et al., 2001). Based on these data as well as other studies, for initial therapy we usually expect the gram-negative enteric bacteria to be susceptible to fluoroquinolones and aminoglycosides. An extended-spectrum cephalosporin (second- or third-generation cephalosporin) usually is active against enteric-gram negative bacteria, but will not be active against Pseudomonas aeruginosa. If the organism is a Pseudomonas aeruginosa, inherent resistance to many drugs is common, but it may be susceptible to fluoroquinolones, aminoglycosides, or extended-spectrum penicillin such as ticarcillin or piperacillin. When administering a fluoroquinolone to treat Pseudomonas aeruginosa, the high-end of the dose range is suggested. Of the currently available fluoroquinolones (human or veterinary drugs), ciprofloxacin is the most active against Pseudomonas aeruginosa.
BACTERIAL SUSCEPTIBILITY TESTING
Bacterial susceptibility to drugs has traditionally been tested with the agar-disk-diffusion test (ADD), also known as the Kirby-Bauer test. With this test, paper disks impregnated with the drug are placed on an agar plate and the drug diffuses into the agar. Activity of the drug against the bacteria correlates with the zone of bacterial inhibition around the disk. The inoculation variables must be well controlled and the test must be performed according to strict procedural guidelines (Lorian, 1996). The precise incubation time (usually 18 to 24 hours), selection and preparation of the agar, and interfering compounds should be known. The ADD test results are only qualitative (that is, it determines only resistant vs. sensitive) rather than providing quantitative information. If this test is performed using standardized procedures, it is valuable, even though it may sometimes overestimate the degree of susceptibility.
It is becoming more common for laboratories to directly measure the minimum inhibitory concentration (MIC) of an organism with an antimicrobial dilution test. The test is usually performed by inoculating the wells of a plate with the bacterial culture and dilutions of antibiotics are arranged across the rows. The MIC can be directly determined by observing the exact concentration required to inhibit bacterial growth. In some laboratories, other methods to measure the MIC are being used such as the E-test (epsilometer test) by AB Biodisk. The E-test is a quantitative technique that measures the MIC by direct measurement of bacterial growth along a concentration gradient of the antibiotic contained in a test strip.
Resistance and susceptibility are determined by comparing the organism's MIC to the drug's breakpoint as established by the National Committee for Clinical Laboratory Standards (NCCLS). (Watts et al., 1999) If bacteria have a MIC equal to, or below the “susceptible” breakpoint, the organism is susceptible. An MIC equal to, or above the “resistant” breakpoint indicates that the organism is resistant regardless of the dose administered or location of the infection. An MIC in the “intermediate” range (or the “F category,” which has been used for enrofloxacin) means that the organism is resistant to the drug unless dosing modifications are used, or unless the drug concentrates at the site of infection, as with topical treatment or when treating bacteria that are isolated from a lower urinary tract infections. MIC tests are more quantitative than an ADD test, but must be performed according to strict guidelines (Lorian, 1996). In some cases, even when the breakpoint is below the “susceptible” range, the organism is resistant in vivo. Examples include cephalosporins for treating oxacillin-resistant staphylococci, and ampicillin for treating β-lactamase producing staphylococci.
PENETRATION TO THE SITE OF INFECTION
For most tissues, antibiotic drug concentrations in the serum or plasma, approximate the drug concentration in the extracellular space (interstitial fluid). This is because there is no barrier to drug diffusion from the vascular compartment to extracellular tissue fluid (Nix et al., 1991). Pores (fenestrations) or microchannels in the capillaries are large enough to allow drug molecules to pass through unless the drug is highly protein bound in the blood. Most antibiotics exhibit low plasma protein binding and this has not been demonstrated to be a factor to impede diffusion of the commonly used antibiotics in veterinary medicine. Tissues lacking pores or channels may inhibit penetration of some drugs (discussed below).
Diffusion Into Tissues
Diffusion of most antibiotics from plasma to tissues is limited by tissue blood flow, rather than drug lipid solubility. This has been called “perfusion-rate limited” drug diffusion. If adequate drug concentrations can be achieved in plasma, it is unlikely that a barrier in the tissue will prevent drug diffusion to the site of infection as long as the tissue has an adequate blood supply. Rapid equilibration between the extracellular fluid and plasma is possible because of the high surface area: volume ratio (high SA:V) demonstrated for most tissues. That is, the surface area of the capillaries is high relative to the volume into which the drug diffuses. Drug diffusion into an abscess or granulation tissue is sometimes a problem because in these conditions, drug penetration relies on simple diffusion and the site of infection lacks adequate blood supply. In addition, low drug concentrations, or slow equilibration in an abscess occurs because of the low surface area:volume ratio (low S:V ratio), rather than a physical barrier to diffusion.
In some tissues, a lipid membrane (such as tight junctions on capillaries or continuous basement membrane) presents a barrier to drug diffusion. This has been called “permeability-rate limited” drug diffusion. In these instances, a drug must be sufficiently lipid-soluble or be actively carried across the membrane in order to reach effective concentrations in tissues. These tissues include: the central nervous system, eye, and prostate. There also is a barrier between plasma and bronchial epithelium (blood:bronchus barrier). This limits drug concentrations of some drugs in the bronchial secretions and epithelial fluid of the airways. Lipophilic drugs may be more likely to diffuse through the blood-bronchus barrier and reach effective drug concentrations in bronchial secretions.
Most bacterial infections are located extracellular, and a cure can be achieved by attaining adequate drug concentrations in the extracellular (interstitial) space rather than intracellular space. However, intracellular infections present another problem. For drugs to reach intracellular sites, they must be carried into the cell or diffuse passively. If cell penetration relies on passive diffusion, only lipid-soluble drugs will be able to diffuse through the cell membrane. Intracellular organisms such as Brucella, Chlamydia, Rickettsia, Bartonella, and Mycobacteria are examples of intracellular pathogens. Staphylococci may become resistant to treatment because of intracellular survival. Examples of drugs that accumulate in leukocytes, fibroblasts, macrophages, and other cells are fluoroquinolones, lincosamides (clindamycin, lincomycin), macrolides (erythromycin, clarithromycin), and the azalides (azithromycin) (Pasqual, 1995). ß-lactam antibiotics and aminoglycosides do not reach effective concentrations within cells. Tetracyclines such as doxycycline, and the class of fluoroquinolones are frequently administered to treat Rickettsia and Ehrlichia infections. There is good evidence for efficacy of doxycycline or fluoroquinolones (enrofloxacin is the only one tested) for treating Rickettsia, but only doxycycline should be considered for its efficacy for treating canine ehrlichiosis.
LOCAL FACTORS THAT AFFECT ANTIBIOTIC EFFECTIVENESS
Local tissue factors may decrease antimicrobial effectiveness. For example, pus and necrotic debris may bind and inactivate vancomycin or aminoglycoside antibiotics (gentamicin or amikacin), causing them to be ineffective. Cellular material also can decrease the activity of topical agents such as polymyxin B. Foreign material in a wound (such as material surgically implanted) can protect bacteria from antibiotics and phagocytosis by forming a biofilm (glycocalyx) at the site of infection (Habash and Reid, 1999). Cations such as Mg++, Al+3, and Ca++ at the site of infection can adversely affect the activity of antimicrobials. Two important drug groups diminished in activity by cations are fluoroquinolones and aminoglycosides. (Cations such as magnesium, iron, and aluminum also can inhibit oral absorption of fluoroquinolones.)
An acidic environment of infected tissue may decrease the effectiveness of clindamycin, erythromycin, fluoroquinolones, and aminoglycosides. Penicillin and tetracycline activity is not affected as much by tissue pH, but hemoglobin at the site of infection will decrease the activity of these drugs. An anaerobic environment decreases the effectiveness of aminoglycosides because oxygen is necessary for drug penetration into bacteria.
As mentioned previously, an adequate blood flow is necessary to deliver an antibiotic to the site of infection. Effective antibacterial drug concentrations may not be attained in tissues that are poorly vascularized (e.g., extremities during shock, sequestered bone fragments, and endocardial valves).
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