Antibiotics – drug classes and mechanisms
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There are many classes of antibiotics. It is useful to know at least a handful of antibiotics, and what they are useful for.  Below we attempt to summarise the main types of antibiotics, their mechanisms and their common uses.

 

Antibiotic Classification and Mechanisms – Click for full size

Table of antibiotics classes, mechanisms and common uses. Group Examples Action Side effects Uses / Info Resistance Broad G+ G- Anaerobes Sulphonamides folic acid inhibitor Sulfasalazine, sulfadiazine Static Nausea, vomiting headaches, hypersensitivity, bone marrow depression, hepatitis Rarely used due to resistance. Allergic reactions common Useless in infections with ‘pus’ Widespread ? ? ? ? Trimethoprim folic acid inhibitor Trimethoprim Static Nausea, vomiting, skin rashes, megoblastic anaemia resulting from folate deficiency (give supplements) May be used in combination with sulphonamide (co-trimoxazole) – particularly in pneumonia. Used alone in RTI and UTI Moderate Y Y Y ? Penicillin β-lactam Penicillin, benzylpenicillin, amoxicillin, flucloxacillin Cidal Few. hypersensitivity (1-10%), nausea, vomiting, encephalopathy (rare – due to renal impairment) Most effective when given IV, can still be given orally. Often given to treat an as-yet unidentified infection. Very short HL’s. Varies (β-lactamase) Y – very! Y Y ? Cephalosporins Modified β-lactam Cefuroxime, cephalexin, cefotaxime Cidal Hypersensitivity, nephrotoxicity, diarrhoea, skin rashes, headache These are synthesised. There are ‘3 generations’. 1st generation not very widely used (resistance). Of those allergic to penicillns, 10% show cross allergy to cephalosporins Varies Y Y Y* ? Monobactam Modified β-lactam Aztreonam Cidal Used against Gram negative bacteria in people with penicillin allergy. Must be given IV or IM ? N N Y N Carbapenems Modified β-lactam Imipenem Cidal Same as penicillins. Neurotoxicity at high concentrations Very wide spectrum ? Y Y Y Y Tetracyclines t-RNA inhibitor Tetracycline, doxycycline Static GI upset if not taken with enough water. Photosensitivity used in renal impairment as an alrternative to β – lactams used a prophylaxis for malaria Widespread Y Y Y ? Aminoglycosides Inhibit cell wall synthesis Gentamycin, streptomycin, neomycin Cidal Can cause sensori-neuro deafness – both in the adult, and in the foetus if given to a pregnant mother. Can also effect balance Resistance can be overcome if used with penicillin or vancomycin. Useful in treating serious gram-positive infections Widespread Y Y Y** N Glycopeptides Inhibit cell wall synthesis Vancomycin, teicoplanin Cidal Nephrotoxicity, rashes, blood disorders, nausea Generally only used for serious staphylococcal infection or IE. Also effective against C. difficile Uncommon Y N ? Macrolides Inhibit translocation of proteins Erythromycin, clarithromycin Cidal / static GI disturbance, hypersensitivity, skin rashes. Generally good against G+, but not against G-. exceptions include H. influenzae and legionella Moderate Y Y N** ? Fusidic Acid Inhibits protein synthesis Fucidic acid GI disturbance, skin eruptions, jaundice Narrow spectrum, used mainly against staphylococci. Most commonly used topically on lesions and conjunctivitis. Occurs rapidly when drug is used alone N Y N ? Quinolones Inhibits DNA transcription Ciprofloxacin, levofloxacin, ofloxacin Cidal Few. GI disorders, skin rashes. dizziness and headaches. Don’t use in conjunction with theophylline (asthmatics)! These are useful against Gram-positive bacteria that are resistant to penicillins. Ciprofloxacin is also good against gram-negative cocci. Generally better against G- Uncommon Y (varies with drugs) Y Y N Metronidazole Inhibits DNA synthesthis, breaks down DNA Metronidazole, tindazole Cidal Nausea, vomiting, metallic taste, intolerance to alcohol, rashes It is equally effective against dividing and non-dividing cells. mainly used to treat C. difficile – that has often been brought on by other AB use Uncommon, but growing ? Y ? Y – not effective against aerobic bacteria! Nitrofurantoin Disrupts DNA Nitrofurantoin Cidal Peripheral neuropathy, GI upset, long term use can cause lung fibrosis Effective against most G+ cocci, and against E.coli (G-). Often used for LUTI’s Uncommon Y N Tetra-cyclines Inhibit protein synthethesis Doxycycline, minocycline, oxytetracycline Static Nausea, vomiting, diarrhoea, discolouration of the teeth in children, intracranial hypertension They are very broad spectrum, and also often used in acne, chlamydia^, cholera, mycoplasma G+ common G- moderate Y Y Y Chlorampenicol Inhibits protein synthesis Chloramphenicol Static Highly Toxic. Bone marrow toxicity, neuritis, headache, rashes, grey baby syndrome Highly toxic – and thus generally reserved for life-threatening conditions. Also used topically for conjunctivitis Moderate Y Y Y ? Lyncosamides Inhibit protein synthesis Clindamycin Cidal/static Nausea, vomiting, rashes, jaundice, neutropaenia, thrombocytonpaenia (bone marrow suppression) Used as prophylaxis for IE in penicillin allergy. Used for strep. Bone infections Moderate ? Y Y • *Gram-negative’s produces a different β-lactamase to Gram-positives; thus cephalosporins are still vulnerable to this (but not to the one produced by Gram-positives). • **only against some - ^chalmydia is not G+ or G-; it is its own thing, as is mycoplasma. Mycoplasma has no cell wall, and thus is unaffected by the β – lactams, as they inhibit cell wall synthesis

 

 

Gram Positive and Gram Negative Bacteria – Click for full size

Summary of Gram Negative and Gram positive Bacteria. Gram-Positive Gram-negative Bacteria Commonly found in Common treatment Bacteria Commonly found in Common treatment Staphylococci & streptococci Skin and wound infections Vancomycin, Teicoplanin, Gentamicin (staphylococci only). Resistant to: Cephalosporins, metronidazole (except clostridia) The Coliform bacteria; E. Coli, Klebsiella, enterobacter, salmonella The Gut! UTI, also can cause ventilator assisted pneumonia, wound infection, biliary tract infection, septicaemia Cephalosporins, gentamicin, ciprofloxacin, tazocin, imipenem, trimethoprim Resistant to: amoxicillin, Staph, Enterococci, corynebacteria Line related infection Pseudomonas Moist environments – chronic leg ulcers. Catheters, pneumonia, septicaemia, CF/bronchiectasis Aminoglycosides, cipro, tazocin, imipenem Resistant to: Most other AB’s Clostridia Gangrenous wound infections, abdominal infections Bacteroids – anaerobic bacteria Intra-abdominal infections, soft tissue infection below the waist Metronidazole, co-amoxiclav, imipenem, Tazocin, clindamycin Resistant to: Benzylpenicillin, amoxicillin, cefuroxime, gentamicin, quinolones, macrolides

 Drugs used in TB
Rifamycins
Inhibits DNA transcription
Rifamycin, rifabutin
Cidal
Nausea, anorexia, pseudomembranous colitis, hepatotoxicity, orange colouration of excreted bodily fluids, toxicity syndromes, drug interactions
Also used in mycobacterial infections – these most commonly occur in those with HIV. Resistance prevents more widespread use
Widespread – develops rapidly
Y
?
?
?
Isoniazid
Inhibits synthesis of cell wall
Isoniazid
Cidal / static
Nausea, vomiting, constipation, peripheral neuropathy, hepatitis, SLE-like-symptoms
Bactericidal on dividing organisms, static on resting. Only effective against myobacteria
Occurs rapidly if used alone
N
N
N
?
Pyrazinamide
Lowers intracellular pH, disrupting synthesis of fatty acids
pyrazinamide
Cidal
Hepatotoxicity, nausea, vomiting, arthralgia, sideroblastic anaemia
Only effective against myobacteria
Occurs rapidly if used alone
N
N
N
?
Ethambutol
Interferes with cell wall synthesis
Ethambutol
Static
Optic neuritis – resulting in red/green colourblindness. neuritis
Only effective against myobacteria
Uncommon
N
N
N
Mycobacteria
This is its own genus of bacteria (like G+ or G-). The group include TB and leprosy. They are acid fast. They are also aerobic.
 
Acid fast
This basically means the organisms are difficult to stain using normal staining techniques. The name refers to the fact they can’t be stained by normal acid (ethanol) staining techniques. These bacteria are often particularly difficult to culture and identify; e.g. TB takes around 6-8 weeks.
‘Sore Throat’
If you are going to give antibiotics for a suspected tonsillitis, don’t give amoxicillin. If the infection is glandular fever, then the amoxicillin can cause a widespread skin reaction. The antibiotics of choice for tonsillitis is penicillin 500mg QID PO.

 

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Dr Tom Leach

Dr Tom Leach MBChB DCH EMCert(ACEM) FRACGP currently works as a GP and an Emergency Department CMO in Australia. He is also a Clinical Associate Lecturer at the Australian National University. After graduating from his medical degree at the University of Manchester in 2011, Tom completed his Foundation Training at Bolton Royal Hospital, before moving to Australia in 2013. He started almostadoctor whilst a third year medical student in 2009. Read full bio

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