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ANTIMICROBIALS
CHAPTER 10
◦ Antimicrobials/antibiotics: drugs that kill or inhibit the growth of microorganisms
(bacteria, protozoa, viruses, fungi).
◦ -________: drugs that kill microorganisms. A must for people/animals with
compromised immune systems.
◦ -________: drugs that inhibit replication but do not directly kill the
microorganism. Once the drug is removed, the organism can multiply again. Rely
on body’s immune system to kill the drug.
NOTE:/disinfectants and antiseptics are antimicrobials that are applied to surfaces
(of the body or an inanimate object)
Terms
-Kill or disable pathogens without killing the host
-The microorganism must be susceptible to the antimicrobial
-The antimicrobial must be able to reach the site of infection in high
concentrations to kill or inhibit the microorganism
-The animal must be able to tolerate high concentrations of the drug
-Convenience and cost also effect drug selection
Rules of antimicrobial therapy
1) Obtaining a sample from the infection site
2) Growing the bacteria that were present in the infected area
3) Determining which antimicrobials the bacteria are affected
by.
◦ The antimicrobial must reach the infection site in
concentrations high enough to exceed the MIC for it to be
effective against the bacteria
◦ It is always a good idea to swab for culture and sensitivity!
Culture and Sensitivity
oMeasures the susceptibility of a bacterial
strain to the effects of an antimicrobial.
oThe MIC is the LOWEST concentration of drug
needed to inhibit bacterial growth.
◦If a bacterial strain has an MIC that is much
lower than the drug’s Maximum Tolerated
Dose, the bacteria are SENSITIVE to the drug
MIC- minimum inhibitory concentration
◦If the MIC is higher than the drug’s Maximum
Tolerated Dose, the bacteria are RESISTANT to
the drug
◦One antimicrobial is not effective against all
strains of bacteria.
◦One strain of bacteria is not affected by all
antimicrobials.
MIC- minimum inhibitory concentration
-Definition: microorganisms developing the ability to survive in the
presence of antimicrobials that should kill them
◦ Mutations provide bacteria with a mechanism to overcome the effect of
antimicrobials that would normally destroy or inactivate them
◦ When resistant bacteria are able to survive after being exposed to a drug and the
bacteria are able to replicate, a problem of a drug-resistant colony arises.
◦ Once resistant, a bacterium will pass on its resistance to its offspring
Drug Resistance
We can try to avoid resistance by:
◦ Administering the correct dose at correct intervals for
the appropriate amount of time and in the correct way.
◦ Educating clients on the importance of adhering to
instructions for medications. ALWAYS FINISH THE FULL
AMOUNT OF ANTIMICROBIALS PRESCRIBED. EVEN IF THE
CONDITION HAS RESOLVED!
◦Cross resistance: when a bacterial strain is resistant to
not only one drug, but other drugs in the same class
Drug Resistance
Definition: the presence of a drug, chemical, or its metabolites in animal tissues or
food products.
◦ Use of drugs in animals intended for food must be withdrawn a specific number of
days before the animal is slaughtered or the food products are sold.
◦ Most residues are not eliminated by cooking or pasteurization!
◦ Exposure to residues in food can lead to an allergic reaction in a person and/or the
growth of resistant bacteria.
Educate food animal producers and clearly write the withdrawal times on prescribed
medication!
Residues
Antimicrobials usually attack a microorganism at these sites: cell wall, cell
membrane, ribosomes, critical enzymes/metabolites, nucleic acids
◦ Interfere with the ability to form its cell wall during cellular division.
◦ Damage the cell membrane during or after cell division.
◦ Interfere with protein synthesis by attacking ribosomes used during translation.
◦ Interfere with enzymes needed for metabolism in the bacterium.
◦ Attack RNA and DNA which is needed for cellular division and protein synthesis.
It is important that the antimicrobial not effect the DNA of the mammal.
How antimicrobials work
-Commonly used, especially penicillin G
-end in –cillin
-Bactericidal against most gram-positive bacteria
-Interfere with bacterial cell wall development (blocks enzymes that
build the cell wall), making the bacterium susceptible to lysing.
PENICILLINS
◦Different types of penicillins exist because they work against the
different enzymes that are used in the construction of the cell wall
(explains how efficacy can vary within the class of Penicilllns)
◦Penicillins are only effective against actively dividing bacteria.
◦ Therefore a penicillin (and other antimicrobials that affect cell
division) should not be given at the same time as a bacteriostatic
drug!
◦*MRSA has enzymes that penicillin can’t bind to.*
PENICILLINS
ALLERGIC REACTIONS are common, ranging from rashes to anaphylactic shock. The
more severe reactions are usually after an injectable penicillin has been
administered. Avoid all penicillins if animal reacts to one of them. Mark clearly in
chart!
Penicillin is absorbed well from injection sites and the GI tract.
◦ Exception is Penicillin G. It is inactivated by stomach acid. Only administer via injection.
PENICILLIN FACTS
-Staphylococcus bacteria produce enzymes (beta lactamases) that break down the beta lactam
ring found within the structure of penicillin.
◦ Oxacillin, cloxacillin, dicloxacillin are resistant to beta lactamase. Downside is they have a
narrow spectrum of activity.
-Potentiated penicillins are chemically combined with another compound that inactivates beta
lactamases. They protect the beta lactam ring, keeping the penicillin intact.
◦ Clavulanic acid (Augmentin, Clavamox) and sublactam are examples of substances added to
penicillin.
PENICILLIN'S SPECTRUM OF ACTIVITY
-RABBITS, GUINEA PIGS, FERRETS, HAMSTERS
◦ Beneficial bacteria may be destroyed by oral penicillin, allowing pathogenic
bacteria to flourish. Severe diarrhea may cause death in the animal.
◦ Also use caution in snakes, birds, turtles, and chinchillas
-Dairy cattle are often administered penicillin to treat mastitis. Milk must be tested
for residues and withdrawal times (milk-out times) must be followed to avoid
contaminated milk.
PENICILLIN PRECAUTIONS
Penicillin G: natural penicillin given via injection b/c it is inactivated by
stomach acid. Available in three forms:
◦ Aqueous solution with potassium or sodium added. Can be given IV.
◦ Suspension with procaine added. DO NOT GIVE IV.
◦ Suspension with benzathine added. DO NOT GIVE IV.
◦ Adding anesthetics delays absorption, extending the duration
◦ DON’T GIVE CLOUDY PENICILLIN (Suspensions) IV!!!
SPECIFIC PENICILLINS
Ampicillin and amoxicillin: effective against gram negative bacteria as they can bind
to the bacteria’s cell wall
◦ Amoxicillin is often combined with clavulanic acid to add protection against beta lactamases
Cloxacillin, dicloxacillin, and oxacillin are naturally resistant to beta lactamases.
They have a narrow spectrum of activity.
SPECIFIC PENICILLINS
-Very similar to Penicillin in structure and mechanism of action
-Have a beta lactam ring, but less affected by beta lactamase producing bacteria
than penicillins are
-Bactericidal
-Bind and inhibit enzymes needed for cell wall formation (like penicillins)
-Bacteria must be dividing for cephalosporins to work
-More effective against Staphylococcus than penicillin
-If patient is allergic to penicillin, cephalosporins are usually avoided (and vice
versa)
-ceph (pre 1975)- or cef- prefixes
CEPHALOSPORINS
Cephalosporins are classified by four generations.
◦ 1st: primarily effective against gram positive bacteria (staph and strep)
◦ cefadroxil (Cefa-drops), cephaphirin (Cefa-Dri), cefazolin (Kefzol),
cephalexin (Keflex)
◦ 2nd: more effective against gram negative bacteria than 1st generations ,
but a little less effective against gram positive than 1st generation
◦ Rarely used generation in vet med.
SPECIFIC CEPHALOSPORINS
◦ 3rd: limited gram + activity, high activity against Pseudomonas and other
gram negative bacteria
◦ cefpodoxime (Simplicef), ceftiofur (Naxcel), cefovecin (Convenia)
SPECIFIC CEPHALOSPORINS