Antimicrobials CHAPTER 10-1 Dr. Dipa Brahmbhatt VMD MpH

Objectives Mechanisms of action of antibiotics Adverse drug reactions and how to mitigate these Selection of antibiotics: Ab resistance, drug absorption, distribution, location of bacteria and drug elimination Antifungal drugs: Advantages, Disadvantages and side effects

Basic Terminology An antimicrobial is a chemical substance that has the capacity, in diluted solutions – To kill (biocidal activity) OR E.g. bactericidal effective in immunocompromised patients Cephalosporins, Penicillin family, Aminoglycosides, fluoroquinolones – Inhibit the growth (biostatic activity) of microbes Functional immune system Sulfonamides, tetracycline, lincosamides/clindamycin. chloramphenicol Staphylococcus

Basic Terminology Antimicrobials can be classified as: – Antibiotics – Antifungals – Antivirals – Antiprotozoals – Antiparasitics – Ch 12

Pathogenic Microorganisms Cause a wide variety of infections and illness in different organs or body systems May be classified as local or systemic – A localized infection may involve skin or an internal organ and may progress into a systemic infection – A systemic infection involves the whole animal and is more serious than a local infection

Pathogenic Microorganisms Enter the body through: Integumentary – Break through skin or mucous membrane Respiratory system – Inhaling contaminated droplets GI tract – Ingestion contaminated food and water Genitourinary – Contaminated vaginal secretions, semen or urine

Antimicrobials Prokaryotic – Fewer organelle & smaller than Euk. – Cell wall +/- capsule – Cell/ plasma membrane – Nucleoid – Cytoplasm – Ribosomes – +/-: pili, plasmids, flagellum – Few have endospore Eukaryotic – Cell / plasma membrane – Nucleus – Nucleolus – Cytoplasm – Mitochondria – Golgi apparatus – Endoplasmic reticulum – Ribosomes – Vacuoles – Lysosomes

Boundary/ semi-permeable Controls cellular activity Genetic material Nucleolus Produces RNA that Forms ribosomes Semi-fluid medium with organelles Energy producers Chemical processor of the cell Has ribosomes Syn. proteins No ribosome Syn. Lipids and some carbohydrates Site of protein synthesis Small, membrane bound Organelles with food, water or metabolic waste Digestive system of the cell

Not membrane bound Smaller than Eukaryotic Barrier against phagocytosis Hairlike protein extensions Attach/communicate with Other cells DNA outside chromosome Replicate independently Pass info via pilus Mobility FEW BACTERIA HAVE ABILITY TO PRODUCE ENDOSPORES Persists in variable environment

Antibiotics spectrum of action Antibiotics work only on bacteria and are described by their spectrum of action (range of bacteria for which the agent is effective) either – Narrow-spectrum antibiotics work only on either gram-positive or gram-negative bacteria (not both) both – Broad-spectrum antibiotics work on both gram-positive and gram- negative bacteria (but not necessarily all) bactericidalbacteriostatic Antibiotics can be classified as bactericidal or bacteriostatic kill – Bactericidals kill the bacteria inhibit the growth – Bacteriostatics inhibit the growth or replication of bacteria

EFFECTIVE ANTIBIOTIC Kill/ disable pathogen w/o affecting the host – Bacteria sensitive to antibiotic – Animal can tolerate high doses of antibiotic – Get’s into infection site at high enough doses Challenging e.g. bacteria in brain (listeria) however it is difficult for penicillin to cross the blood-brain- barrier Drug Selection – Client compliance: ease of administration convenient dose schedule and form – cost

Gram stain Procedure Staphlylococcus sp. Streptococcus sp. Salmonella sp. Proteus sp.

Agar Diffusion Test A.k.a. Kirby-Bauer antibiotic sensitivity testing Used to determine if a particular antibiotic is effective against a particular bacterium. Antibiotic-impregnated disks placed onto agar plates containing bacteria being tested. After incubation at proper temperature for the proper time, zones of inhibition (clear zones) are measured and compared to a standardized chart to determine R. – R = resistant (antibiotic does not work): high conc. causes significant SE in host – I = intermediate (antibiotic may work) – S = sensitive (antibiotic will work)

Broth Dilution Method Minimum Inhibitory Concentration (MIC): Lowest conc. Of Antibiotic that visually inhibits growth of bacteria High MIC = resistant

Considerations when using antibiotics Antibiotic resistance – Means that the bacteria survive and continue to multiply after administration of the antibiotic – Occurs when bacteria change in some way that reduces or eliminates the effectiveness of the agent used to cure or prevent the infection – Causes Drug not properly used (used for viral infection) Not administered for proper length of time Not at proper dose

Considerations when using antibiotics Resistance develops by: Natural/Intrinsic resistance – Ab. Acts on enz. Systems or biological processes not used by microorganism Acquired resistance: sensitive before – Bacterial mutation » E.g. bacteria produce beta-lactamase, inactivates penicillin – Bacteria acquiring genes that code for resistance » R (resistance) Plasmids Other means » Cross –resistance e.g. between Ab’s. with same actions: penicillins & cepalosporins » Noscomial infections (sick patients and close contact) » Handwashing

Considerations when using antibiotics – Responsibility Administer appropriate dose – interval – time – manner Client education – Follow instructions – Finish medication

Considerations when using antibiotics An antibiotic residue is the presence of a chemical or its metabolites in animal tissue or food products Residue is not usually degraded by cooking/ pasteurization – Antibiotic residues can cause allergic reactions in people or can produce resistant bacteria that can be transferred to people who consume these products – Withdrawal times for antibiotics are aimed at eliminating antibiotic residues in food-producing animals

Considerations when using antibiotics The FDA approves all drugs marketed for use in animals in the United States The FDA also establishes tolerances for drug residues to insure food safety The FDA also establishes withdrawal times and withholding periods – Times after drug treatment when milk and eggs are not to be used for food, and also when animals are not to be slaughtered for their meat – FARAD: Food Animal Residue Avoidance Databank

How Do Antibiotics Work? Antibiotics work by a variety of mechanisms: 1) Inhibition of cell wall synthesis Bacteria have cell walls & animals don’t Bactericidal: growing and dividing not in developed bacteria 2) Damage to the cell membrane Alters membrane permeability Bactericidal/static Loss of cellular substances causes lysis of cell

How Do Antibiotics Work? 3) Inhibition of protein synthesis In ribosomes – RNA copy (of DNA), t RNA (AA’s) – Disrupts linkage of AA’s – Bactericidal/static 4) Interference with metabolism Block enzymes / essential nutrients – Can’t divide and cell death Bacteriostatic 5) Impairment of nucleic acids Intefere with nucleic acid production – Can’t divide/ function properly Bactericidal

-cidal Penicillin Cephalospo rins -cidal quinolones -static sulphonamides Block enzymes -static/cidal Polymyxin B (TAO) -static/cidal Lincosamides Macrolides Tetracyclines Aminoglycocides

Classes of Antibiotics Cell wall/ membrane agents Protein synthesis agents Antimetabolites Nucleic acid agents Miscellaneous agents

Cell Wall Agents Penicillins Penicillins beta-lactam – Have beta-lactam structure that interferes with bacterial cell wall synthesis – Suffix –cillin – Spectrum of activity depends on the type of penicillin mostly gram + – Effective against actively growing bacteria (cell division – cell wall) – Bacteriostatic should not be used/ used judiciously at same time as bactericidal drug – Hydrophilic at blood pH hence difficult to reach therapeutic levels: eye, brain, prostate – Excreted as a whole in urine. Hence good for UTI, kidney, bladder, genitourinary tract – Cross – resistance: resistance to one Ab in group than probably to other as well – Pseudomonas – otitis externa is resistant to penicillin

Cell Wall Agents Natural Penicillins Natural Penicillins – Penicillin G and V are narrow-spectrum gram-positive antibiotics (Staphylococcus sp. & Streptococcous sp.) Penicillin G is given parenterally (inactivated by stomach acid) a) Aqueous solution (only one given IV): Only sodium or potassium salt of Pen-G can be admin. IV, IM, SQ: peak levels 20 mins – Refrigerated: 14 days b) Suspension: Procaine & Benzathine : increase duration. IM/SQ – PPG: 24 hr. duration – Benzathine PG: 5 days – Peak plasma not as high as aqueous – SE: Anorexia, V/D (orally), hypersensitivity

Cell Wall Agents – Aminopenicillins: Penicillin V is given orally – Give PCN on empty stomach (except amoxicillin) – Penicillin V potassium: preferred better absorbed in GI and stable in stomach acid – Broader-spectrum penicillins are semi-synthetic Slowly excreted from kidneys Examples include amoxicillin (ok with food), Don’t give with food: ampicillin (POLYFLEX SUSPENSION), carbenicillin, ticarcillin, and methicillin More gram – than Pen G

Cell Wall Agents Beta-lactamase resistant penicillins More resistant to beta-lactamase / penicillinase (an enzyme produced by some bacteria; Staphylococcus that destroys the beta-lactam structure of penicillin) Examples include methicillin, oxacillin, dicloxacillin, cloxacillin, and floxacillin – narrow spectrum against beta lactamase Bovine staphylococcal mastitis, staphylococcal osteomyelitis, staphylococcal pyoderma, – Potentiated penicillins are chemically combined with another drug to enhance the effects of both amoxicillin and clavulanic acid (from streptomyces) / Augmentin An example is a drug containing amoxicillin and clavulanic acid (from streptomyces) / Augmentin (which binds to beta-lactamase to prevent the beta-lactam ring from being destroyed)

Penicillins - Considerations Hypersensitivity Anaphylactic reactions Injectable products Aggressive therapy, epinephrine, corticosteroids Milder: skin rashes/ urticaria, swelling – LNs, eosinophilia/ neutropenia, fever Cross-reactivity to other drugs in family MAKE NB OF THIS CLEARLY ON RECORD Destroy normal flora: superinfection/ suprainfection Severe diarrhea and death: Guinea pigs, ferrets, hamsters and rabbit use with caution snakes, birds, turtles, chinchillas Withdrawal time for milk and meat

Cell Wall Agents Cephalosporins Cephalosporins – Are semi-synthetic, broad-spectrum antibiotics that are structurally related to the penicillin's, work on rapidly growing bacteria, cephalosporinases bacterial beta lactamases Have the beta-lactam ring ceph- or cef- prefix in the drug name Not TOC for brain/ eye/ prostate infections High concentration in urine four generations – Are classified into four generations In general, as the number of the generation increases, the spectrum of activity broadens (but becomes less effective against gram-positive bacteria) 1 st generation: cefadroxil (cefa-tabs, cefa-drops) ; cephapirin (cefa- lak and cefa-dri intramammary infusions); human products: cephalexin >> gram +: Staphylococcus, Streptococcus 2 nd generation: Human product: CEFOXITIN (mefoxin) 3 rd generation: ceftiofur (Naxcel/ excenel); cefpodoxime (Simplicef); human: cefotaxime (claforan). Most injectable except for cefpodoxime

Convenia® (cefovecin sodium) Convenia® (cefovecin sodium) – First antibiotic that provides an assured course of treatment by providing up to 14 days of treatment in a single injection, eliminating missed doses associated with daily oral antibiotic administration – Labeled specifically for secondary superficial pyoderma, abscesses, and wounds (S. intermedius, S. canis ) in dogs and abscesses and wounds (P. multocida) in cats. – Must be reconstituted, refrigerated, and used within 28 days of reconstitution – Second dose may be necessary in 14 days.

Cephalosporins - Precautions Mostly safe, less hypersensitivity reactions then penicillin Suprainfection may have with 1 st generation cephalosporins Oral drugs SE: anorexia, v/d Efficacy reduced with bacteriostatic drugs

References Romich, J.A. Pharmacology for Veterinary Technicians, 2 nd edition Bill, R.L. Clinical Pharmacology and Therapeutics for the Veterinary Technician, 3 rd edition