Antibiotics

Kate Charters

Choosing an antibiotic

Know the group of bacteria: Gram positive, negative or

anaerobic

Know which antibiotics are effective (including in allergic

patients)

Know which diseases require specific knowledge:

pneumonia, meningitis

Consider patient factors: renal failure, discharge to home?

Bacteriology

• Staphylococci

• Streptococci

• Enterococci

• Pseudomonas

• Escherichia coli

• Clostridium

• Bacteroidesfragilis

Penicillins

Interfere with bacterial cell wall synthesis. Poor CSF

penetration. Excreted in urine.

1. Original penicillins

2. Enteric active penicillins

3. Anti-staphylococcal penicillins

4. Anti-pseudomonal penicillins

5. Penicillins with beta lactamase inhibitors

Penicillins

Class Examples Spectrum Notes

Original penicillins Benzylpenicillin

(PO/IM/IV),

phenoxymethylpenicillin

Streptococcus, enterococcus,

some anaerobes

Gram –ves and

staph. are mostly

resistant

Enteric active

penicillins

Amoxicillin (PO/IV),

ampicillin (IV)Enterococcus, streptococcus

(rash if EBV),

some anaerobes

Gram –ves are

mostly resistant

Antistaphylococca

l penicillins

Flucoxacillin/

dicloxacillin (PO/IV)

Exclusive Gram

+ve: Staph &strep.

Antipseudomonal

penicillin

Piperacillin/ ticarcillin

(IV)

Broad spectrum

gram –ve & +ve

Expensive: Use only

when serious gram

neg. sepsis

Penicillin + beta

lactamase inhibitor

Amoxycillin + clavulanic

acid (PO), ticarcillin +

CA (IV)

Broad spectrum

gram –ve & +ve

and anaerobes

Useful if wide range

of pathogens

Penicillins Side Effects

Hypersensitivity: 1-10% of exposed individuals, only 0.05%

get anaphylaxis. Some cross reactivity with

cephalosporins and beta-lactams.

Encephalopathy: Rare – more likely in severe renal failure

Common: Diarrhoea and antibiotic associated colitis

Review questions

Name some penicillins that would work for staphylococcal infection.

What don’t they work on?

What do original penicillins work best on?

What is the drug of choice in enterococcus?

What drug should only be used if serious gram negative sepsis is suspected?

Which penicillins are active against MRSA?

Cephalosporins

Four generations: As you increase the generation, there is less staph/strep coverage and more gram negative.

1st generation: Cephazolin IV, cephalexin PO

2nd generation: Mostly used in ENT

3rd generation: Cephtriaxone IV/IM or cefotaxime IV

4th generation: Covers pseudomonas and staphylococcus

Indications: Penicillin allergy, gram negative cover in renal impairment or pregnancy & meningitis. Useless for enterococci.

Review Questions

What are first generation cephalosporins used for?

What is the name of the oral first generation?

What organisms are ceftriaxone effective against?

Carbopenems

Imipenem and meropenem (IV)

Insanely broad spectrum (except MRSA and ampicillin

resistant enterococci)

Expensive, last resort

Glycopeptides

Vancomycin

Narrow spectrum: First line for MRSA and ampicillin

resistant enterococci

Aminoglycosides

Gentamicin (IV or IM), neomycin, streptomycin

Gram negatives including pseudomonas

Renal toxicity(excreted by kidney) and ototoxicity

Use for a maximum of 7 days as side effects are dose

related

Can measure serum concentration

Macrolides

Erythromycin, roxithromycin, clarithromycin &

azithromycin

Spectrum: Staph/Strep and atypical organisms

(chlamydia, legionella)

Good for allergic patients

Tetracyclines

Doxycycline PO

Spectrum: Mainly atypicals (malaria prophylaxis,

chlamydia, rickettsia)

Deposits in teeth and bones – caution in children under

12

Caution: Hepatic impairment, myasthenia gravis, SLE

Side effects: N/V, diarrhoea, oesophageal irritation

Lincosamide

Clindamycin PO/IV

Spectrum: Staph (not MRSA), strep and anaerobes –

particularly used for staphylococcal joint and bone

infection

Quinolones

Ciprofloxacin PO/IV

Spectrum: Gram negative and atypicals

Newer quinolones (i.e. moxifloxacin) have better gram

positive activity

Trimethoprim and Sulphas

Trimethoprim PO has limited gram positive and negative

– it’s used mainly for UTIs

Trimethoprim w/ sulphamethoxazole (PO/IV) = Bactrim

Drug of choice for Pneumocystis carinii pneumonia and

chest sepsis

Chloramphenicol

Not used in Australia as it suppresses bone marrow – used

commonly in the third world as very broad spectrum ad

penetrates the BBB

Exception: eye drops

Nitroimidazoles

Metronidazole PO/IV

First line: Anaerobes **Clostridium difficile colitis

Summary

•Penicillin

•Cephalosporin

•Carbopenems

•Macrolides

•Clindamycin

•Vancomycin

•Gentamycin

•3/4th gen cephalosporins

•Trimethoprim

•Ciprofloxacin

•Antipseudomonalpenicillin

•Carbopenems

•Metronidazole

•Clindamycin

•Carbopenems

•Some penicillins

•Macrolides

•Doxycycline

•Ciprofloxacin

Pneumonia

What are the common organisms in community acquired

pneumonia?

Which antibiotic would you use?

How would you classify the severity?

Answers

Common pathogens:

Strep pneumonia, mycoplasma pneumonia, chlamydia,

staph, h.influenza, legionella

SMART-COP

0-2: Low risk of

needing intensive

respiratory or

vasopressor support

3-4: Moderate risk (1

in 8)

5-6: High risk (1 in 3)

>7: Very high (2 in 3)

Score of >3 is 92%

sensitive for IRVS

CURB 65

Risk of death at 30 days

increases as score increases.

0-1: Tx as outpatient (0.7-3.2%

risk)

2-3: Consider short stay in

hospital or hospital in the home

(13-17% risk)

4-5: Req. hospitalisation, consider

ICU (41.5-57% risk)

Hospital Acquired Pneumonia

What are the common pathogens?

What is empirical treatment?

HAP

Suspect if purulent sputum, persistant infiltrate on CXR,

increased oxygen requirement, febrile or

leukocytosis/leukopenia.

Colonisation of the oropharynx w/ aerobic gram

negative bacilli and multidrug resistant hospital

pathogens: MRSA, drug-resistant Enterobacteriaceae,

Pseudomonas aeruginosa etc.

Can occur due to atypicals: Legionella, Aspergillus, respiratory viruses

Cellulitis

Organisms?

Antibiotics?

Answers

Staphylococcus, streptococcus

Dicloxacillin/ flucloxacillin

UTI

Organisms?

Treatments?

Pyelonephritis

Answers

E. Coli mostly, then klebsiella, protease

Trimethoprim or cephalexin (don’t know why) or

amoxyl/clavulanic acid

Pyelonephritis – higher dose or gentamicin and

amoxy/amp

Meningitis

Know this algorithm!

That’s all folks!