7 Basic microbiology
SURGICALLY IMPORTANTMICRO-ORGANISMS
Examples of the above are shown in Box 7.1.
Box 7.1 Microbial infections
GRAM POSITIVE COCCI
The important ones are staphylococci and streptococci.
Staphylococci
Staph. saprophyticus, a commensal, may cause urin-ary tract infections in sexually active women.
Streptococci
GRAM NEGATIVE BACILLI
Facultative anaerobes (Coliforms)
Aerobic Gram negative bacilli
Pseudomonas aeruginosa
This inhabits human and animal gastrointestinal tracts, water and soil. The organism survives in moist environments in hospitals and may also survive in aqueous antiseptics and other fluids. It is an important cause of hospital-acquired infections. It particularly affects patients with serious underlying conditions, e.g. burns and malignancy, or as a result of therapeutic interventions, e.g. urinary catheters, endotracheal tubes. It is a frequent cause of infection in the immuno-compromised patient. It is a pathogen in the following conditions:
SPECIFIC ANTIBIOTICS AND ANTIMICROBIALS
PENICILLINS
Co-amoxiclav (Augmentin®)
This contains amoxicillin and potassium clavulanate. It may be administered either orally or i.v. The clavulanate is inhibitory to β-lactamase and extends the spectrum of amoxicillin. It is active against some coliforms, staphylococci and bacteroides. It is also useful in surgery as prophylaxis in bowel, hepatobiliary and GU surgery.
MACROLIDES
Erythromycin
This is usually administered orally or i.v. by slowinfusion. Its use in surgical patients is limited. It is usually used as a second-line drug in patients allergic to penicillin. It is active against streptococci, staphylo-cocci, clostridia and Campylobacter. It is used for skin and soft tissue infections and respiratory tract infections. It is valuable in atypical pneumonia, Legionnaire’s disease and Campylobacter enteritis. The chief side effect when given orally is diarrhoea. When given i.v. phlebitis at the site of infusion is a common side effect. It may potentiate warfarin and cyclosporin.
AMINOGLYCOSIDES
ANTIBIOTICS IN SURGERY
PRINCIPLES OF ANTIBIOTIC THERAPY
Selection of antibiotic
The following sequence of events usually occurs in selection of an antibiotic:
PROPHYLACTIC ANTIBIOTICS
Indications for prophylactic antibiotics
Most prophylactic antibiotics are given to prevent wound infection. In some cases they are given prior to instrumental procedures in potentially infected sites, e.g. when performing cystoscopy, when they are given to prevent bacteraemia. Any patients with congenital heart disease, rheumatic heart disease, or prosthetic valves should be given antibiotics before an elective procedure which may result in bacteraemia. Procedures include dental procedures (including scaling and polishing), GU instrumentation, some types of GI endoscopy, respiratory tract instrumentation and open surgery. In most cases one dose is given preoperatively, either orally if the procedure is under local anaesthetic (1 h preoperatively) or intravenously if the procedure is under general anaesthetic. An additional dose is sometimes given postoperatively. The aim is to achieve therapeutic levels at the time of surgery. Table 7.1 shows some indications for prophylactic antibiotics, the likely organism involved, and a recommended prophylactic regime.
Clinical situation | Likely organism(s) | Prophylactic regime |
---|---|---|
Appendicectomy | Anaerobes | Metronidazole (single dose pr 1 h preop) |
Biliary tract surgery | Coliforms | Cephalosporin (i.v. immediately preop and for 24 h postop) |
Colorectal surgery | Coliforms Anaerobes | Metronidazole + cephalosporin or gentamicin (i.v. immediately preop and for up to 48 h postop) |
GU surgery (open surgery) | Coliforms | Gentamicin (single i.v. dose preprocedure). Cephalosporin (i.v. immediately preop and for 24–48 h post-op) or gentamicin (single i.v. dose immediately preop) |
Insertion of prosthetic joints | Staph. aureus | Flucloxacillin (i.v. immediately preop and for 24–48 h postop) |
Staph. epidermidis | ||
Amputation of limb | C. perfringens | Penicillin (i.v. immediately preop and for 24 h postop) |
Vascular surgery with prosthetic graft | Staph. aureus | Cephalosporin (i.v. immediately preop and for 24 h postop) |
Staph. epidermidis Coliforms | ||
Prevention of tetanus in contaminated wound (+ immunoprophylaxis) | C. tetanus | Penicillin (i.v. or i.m. on presentation) |
Prophylaxis of endocarditis | ||
Minor dental procedure under LA | Oral streptococci | Amoxicillin (single oral dose 1 h preop; clindamycin if allergic) |
Major dental procedure under GA | Low risk: amoxicillin (oral dose 4 h preop and one dose postop) | |
High risk: amoxicillin & gentamicin (i.m. or i.v. immediately preop; vancomycin if allergic) | ||
FGU instrumentation | Enterococci | Amoxicillin + gentamicin (i.v. immediately preop) |