Microbiology

Question 1

Bacteria classification

Answer 1

• shape:
— bacilli – rod shaped; — cocci – spherical;

• Gram staining:
— Gram positive – blue; — Gram negative – pink;

• growth requirements:
— aerobic;
— anaerobic
— facultatively anaerobic

Question 2

Gram positive cocci: Staphylococci

Answer 2

Staphylococci
These tend to be arranged in grape-like clusters. They may be divided into coagulase positive and coagulase negative. Coagulase positive staphylococci are called Staph. aureus. They are responsible for the following:

1. Superficial infections; e.g. boils, abscesses, styes, conjunctivitis, wound infections
2. Deep infection; e.g. septicaemia, endocarditis, osteomyelitis, pneumonia
3. Food poisoning
4. Toxic shock syndrome

Question 3

Gram positive cocci: Coagulase negative staphylococci

Answer 3

Coagulase negative staphylococci, e.g. Staph. epi- dermidis are of lower pathogenicity and rarely cause infection in healthy people. They form part of the nor- mal skin flora. However, they may be responsible for infection in association with foreign bodies, e.g. pros- thetic cardiac valves, intravenous lines, continuous ambulatory peritoneal dialysis, and vascular grafts. These infections may lead to septicaemia and endo- carditis and become life threatening. Their treatment with antibiotic alone is often inadequate, and the pros- thesis may require removal.
Staph. saprophyticus, a commensal, may cause urinary tract infections in sexually active women.

Question 4

Antibiotic sensitivity

Answer 4

Staph. aureus appears in resistant forms, especially in hospital practice. Recently there has been an increase in MRSA (methicillin-resistant Staph. aureus) which is now the predominant hospital strain and presents a major threat to surgical patients. This is resistant to all penicillins and cephalosporins.

Antibiotics that may be active against Staph. aureus include:

• penicillin (80% of hospital strains are resistant);
• flucloxacillin (active against beta-lactamase-
producing organisms but not MRSA);
• erythromycin;
• clindamycin;
• fusidic acid;
• cephalosporins; and
• vancomycin.

Question 5

Streptococci

Answer 5

These are spherical or oval cocci occurring in chains. They are classified by their ability to lyse red blood cells present in blood containing culture medium. They are further subdivided by serology, on the basis of polysaccharide antigens present on their surface, into Lancefield groups. The species responsible for sepsis are the beta-haemolytic strains where colonies completely lyse the blood cells on a culture plate, causing a colourless, clear, sharply defined zone. They include Lancefield groups A, B, C and G.

Question 6

Lancefield Group A

Answer 6

Strep. pyogenes causes:
• tonsillitis and pharyngitis;
• peritonsillar abscess (quinsy);
• otitis media;
• mastoiditis;
• wound infection with cellulitis and lymphangitis; • erysipelas; and
• necrotising fasciitis.
Antibiotic sensitivity Penicillin is the drug of choice. All strains are sensitive. In patients allergic to peni- cillin, erythromycin is the drug of choice, but some strains are resistant.

Question 7

Lancefield Group D

Answer 7

‘Viridans’ streptococci These show alpha haemolysis on blood-containing culture plates with a green (hence the term viridans) discoloration around the colonies. Most human strains are commensals of the upper
respiratory tract and are of low pathogenicity. They are responsible for endocarditis. ‘Strep. milleri’ may be classified with this group but it is now more often clas- sified with pyogenic streptococci. It may cause liver, lung or brain abscesses.
Streptococcus pneumoniae (Pneumococcus) This has a polysaccharide capsule, which is correlated with its virulence, probably because it prevents or inhibits phagocytosis. Eighty-four capsular types are recog- nized. Pneumococci are often paired on gram stain. The organism is responsible for the following
• lobar pneumonia
• chronic bronchitis
• meningitis
• sinusitis
• conjunctivitis
• septicaemia (especially in splenectomised patients).

Antibiotic sensitivity All strains are sensitive to peni- cillin and erythromycin.

Question 8

Enterococci

Answer 8

Enterococcus faecalis is the most surgically important in this group. It may cause urinary tract infections and abdominal wound infections and may be isolated from bile in acute cholecystitis. Enterococci are usually sen- sitive to ampicillin, moderately resistant to penicillin, and resistant to cephalosporins.

Question 9

Gram positive rods

Answer 9

Anaerobic Gram positive rods are mainly soil sapro- phytes but a few are pathogens. The surgically import- ant ones include species which produce powerful toxins, e.g. Clostridium perfringens (gas gangrene), C. tetani (tetanus), C. botulinum (botulism) and C. difficile (diarrhoea in association with antibiotic- induced colitis).

Question 10

Gram negative bacilli

Answer 10

This is a large group of micro-organisms of surgi- cal importance. They may be divided into facultative anaerobes, e.g. E. coli and Klebsiella, and aerobes, of which Pseudomonas is the most commonly encoun- tered in surgical practice.

Question 11

Facultative anaerobes (Coliforms): E.Coli

Answer 11

Escherichia coli
This is a normal inhabitant of the human intestine. Some strains produce powerful toxins. E. coli is an important cause of sepsis and diarrhoea. Examples of sepsis include:
• UTIs;
• wound infection, especially after surgery on the
lower gastrointestinal tract;
• peritonitis;
• biliary tract infection; and
• septicaemia.
Examples of diarrhoeal illnesses include:
• infantile gastroenteritis;
• traveller’s diarrhea; and
• haemorrhagic diarrhoea, e.g. haemolytic uraemic
syndrome.

Question 12

Facultative anaerobes (Coliforms): Klebsiella

Answer 12

Klebsiella
Klebsiella spp inhabit the human intestine. Some strains are saprophytic in soil, water and vegetation. They are responsible for:
• UTIs;
• septicaemia;
• endocarditis; and • pneumonia (rare).
Proteus
Proteus spp. are responsible for:
• UTIs;
• wound infections, e.g. burns, pressure sores; and • septicaemia.

Question 13

Facultative anaerobes (Coliforms): Salmonella

Answer 13

They inhabit animal gastrointestinal tract. They are pre- dominantly animal pathogens which can cause disease in humans. Salmonella typhi and Salmonella paratyphi differ from other species in that man is the only natural host. Foodstuffs from animal sources are the usual source of transmission of infection. They are responsible for:
• enteric fever, typhoid or paratyphoid; these are due to S. typhi and S. paratyphi A, B, C;
• gastroenteritis (food poisoning), usually due to S. enteritidis or S. typhimurium;
• osteomyelitis (rare); and • septic arthritis (rare).
S. typhi may survive in symptomless carriers and persist in the gall bladder. Faecal carriage may occur by contamination with bile, and epidemics may occur especially if the carrier is a food handler.

Question 14

Facultative anaerobes (Coliforms): Shigella

Answer 14

They are intestinal parasites in man. They cause dysen- tery. Sh. dysenteriae which produces exotoxins causes the most severe illness. Other shigellae may cause a milder form of dysentery, Sh. sonnei being the most common cause in the UK.

Question 15

Facultative anaerobes (Coliforms): Yersinia

Answer 15

Yersinia
These are animal parasites which occasionally cause disease in humans. Yersinia pseudotuberculosis and Yersinia enterocolitica are the most common, causing food poisoning and mesenteric adenitis.

Question 16

Facultative anaerobes (Coliforms): Other enterobacteria

Answer 16

These include enterobacter, citrobacter, providencia, morganella and serratia. They are human and animal intestinal residents but some strains are saprophytes. Moist hospital environments may act as reservoirs. They are often multiresistant to antibiotics. They may cause the following:

• UTIs;
• wound infections after abdominal surgery;
• respiratory infections in hospitalised patients; and • septicaemia.

Question 17

Facultative anaerobes (Coliforms): Antibiotic sensitivity

Answer 17

Since many strains are now resistant to commonly- used antibiotics, sensitivity should be determined. In systemic infection, cephalosporins, gentamicin, ciprofloxacin or carbapenems may be used. In UTIs trimethoprim, amoxicillin and nitrofurantoin may be used for sensitive organisms.

Question 18

Aerobic Gram negative bacilli: Pseudomonas aeruginosa

Answer 18

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:

• UTIs, especially within indwelling catheters;
• burns;
• wound infections;
• septicaemia;
• pressure sores;
• venous stasis ulcers;
• chest infections, especially patients on mechanical
ventilation and those with cystic fibrosis; and
• eye infections (it may contaminate certain types of eye drops).

Question 19

Aerobic Gram negative bacilli: Antibiotic sensitivity

Answer 19

The presence of Ps. aeruginosa is not necessarily an indication for antibiotic therapy especially if it is isolated from a superficial site. Clinical and bacterio- logical assessment in the individual patient is appro- priate before prescribing antibiotics. Ps. aeruginosa is resistant to most common antibiotics. The most suitable antibiotics are aminoglycosides, ciprofloxacin, ceftazidime, and piperacillin/tazobactam.

Question 20

Other Gram negative bacilli: Campylobacter

Answer 20

These are curved or spiral rods which are micro- aerophilic. They are found in various animal species, including chickens, domestic animals and seagulls. Campylobacter is the most common cause of bacterial food poisoning in the UK.

Question 21

Other Gram negative bacilli: Haemophilus influenzae

Answer 21

This is mainly found in the respiratory tract, often as part of the normal flora but may also cause respiratory dis- ease, especially community-acquired respiratory disease. It exists in non-capsulated and capsulated strains.

Non-capsulated strains are responsible for exacer- bation of chronic bronchitis and bronchiectasis. Capsulated strains often cause severe infections in young children, e.g. meningitis, acute epiglottitis, osteomyelitis, arthritis and orbital cellulitis. Septicaemia may occur especially as part of postsplenectomy sepsis. A vaccine is available against H. influenzae type B (HiB).
Antibiotic sensitivity
These are usually sensitive to amoxicillin, tetracycline, cephalosporins (second and third generations) and trimethoprim. Chloramphenicol should be reserved for severe infections, e.g. meningitis and acute epiglottitis.

Question 22

Other Gram negative bacilli: Pasteurella multocida

Answer 22

This is a small ovoid gram negative bacillus. It inhabits the respiratory tract of many animals, notably dogs and cats. In man it may cause septic wounds after animal bites. It is usually sensitive to penicillin, tetra- cycline, erythromycin and aminoglycosides.

Question 23

Penicillins: Benzyl penicillin

Answer 23

This is active against streptococci, pneumococci, clostridia, N. gonorrhoea and N. meningitidis. Few staphylococci are now sensitive. The main surgical indications are for the prophylaxis of gas gangrene and tetanus and for streptococcal wound infections. It may be given parenterally, either i.v. or i.m.

Question 24

Penicillins: Phenoxymethyl penicillin (penicillin V)

Answer 24

This is administered orally. It is used prophylactically following splenectomy to prevent pneumococcal septi- caemia, especially in children where it is used long term. It may also be used for prophylaxis in patients with rheumatic heart disease.

Question 25

Penicillins: Flucloxacillin

Answer 25

This is administered either orally, i.m. or i.v. for penicillinase-resistant Staphylococcus aureus. It is often used as an adjunct to drainage of abscesses, especially in diabetics or immunosuppressed patients.

Question 26

Penicillins: Amoxicillin and ampicillin

Answer 26

These may be administered either orally, i.m. or i.v. Their use in the surgical context is largely for chest infections or urinary tract infections. Many sta- phylococci and coliforms produce β-lactamase and are, therefore, resistant. Amoxicillin and ampicillin are usually active against Enterococcus faecalis and Haemophilus influenzae.

Question 27

Penicillins: Co-amoxiclav (Augmentin®)

Answer 27

This contains amoxicillin and potassium clavulanate. It may be administered either orally or i.v. The cla- vulanate 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, hepatobil- iary and GU surgery.

Question 28

Penicillins: Piperacillin/tazobactam (Tazocin®)

Answer 28

This may be administered i.m. or i.v. It is active against bacteroides, coliforms, klebsiella and Pseudomonas aeruginosa. It is often used in combination with an aminoglycoside for life-threatening infections.
When administering penicillins, care should be taken to check for previous sensitivity. Caution should be par- ticularly exercised in asthmatics and others with a his- tory of allergic conditions. Hypersensitivity reactions are usually manifested by an urticarial rash, although anaphylaxis may occur. Cross-sensitivity occurs between different penicillins. Most penicillins are relatively non- toxic, and, therefore, large doses may be given. Caution must be exercised in patients with renal and/or cardiac failure, as injectable forms contain potassium and sodium salts. Rarely, convulsions may occur after giv- ing high doses i.v. or following intrathecal injection.

Question 29

Carbapenems

Answer 29

These are β-lactam drugs which resist most common β-lactamases and have a very broad spectrum. They should be reserved for ‘difficult to treat’ infections.

Meropenem and imipenem
These are administered by the i.v. route and are indi- cated for respiratory, abdominal and other infections due to resistant gram negative organisms. Imipenem may cause convulsions.

Question 30

Cephalosporins

Answer 30

These drugs are assigned to three generations. Specific examples of each generation in surgical usage are described below. Unfortunately, resistance levels are increasing rapidly.

Cefradine, cefalexin and cefaclor
These are first generation cephalosporins which are usually given orally. They are active against a wide range of Gram positive and Gram negative organ- isms, including E. coli, klebsiella, proteus, and Staph. aureus (unless methicillin-resistant). They are not active against Enterococcus faecalis, Ps. aeruginosa or bacteroides. They are useful as second line drugs for the treatment of urinary tract infections, respiratory tract infections, skin and soft tissue infections.

Question 31

Cephalosporins: Cefuroxime

Answer 31

This is a second generation cephalosporin which may be given orally, i.m. or i.v. In practice it is used most commonly i.v. It is a broad spectrum antibiotic against Gram positive and Gram negative organisms. It is not active against bacteroides or against Pseudomonas. It is widely used in prophylaxis, especially in combin- ation with metronidazole in colorectal and biliary tract surgery.

Question 32

Cephalosporins: Cefotaxime and ceftazidime

Answer 32

These are third generation cephalosporins which are administered i.m. or i.v. They have a broad spectrum similar to second generation drugs and ceftazidime is also active against Pseudomonas. They are normally reserved for use in serious sepsis due to susceptible aerobic Gram negative bacilli.

About 10% of patients who are allergic to penicillin are also allergic to cephalosporins. Rashes and fever may occur. In patients with renal failure, dose reduc- tion is required. Mild transient rises in liver enzymes may occur.

Question 33

Sulphonamides

Answer 33

Co-trimoxazole (sulphamethoxazole trimethoprim)
This may be given either orally or i.v. It is used for treatment of urinary tract infections and respiratory infections. It is active against Gram positive and Gram negative organisms. Ps. aeruginosa is resistant. It may be used for Salmonella septicaemia and Pneumocystis pneumonia. Nausea, vomiting, rashes and mouth ulcers may occur. Leucopenia and thrombocytopenia may also occur occasionally. Life threatening reactions are not uncommon in the elderly.

Question 34

Trimethoprim

Answer 34

This may be administered orally or i.v. by slow infusion. It is used for urinary tract infections and respira- tory infections. It should be avoided in pregnancy. Nausea, vomiting, rashes, stomatitis and marrow sup- pression may occur. It potentiates the action of war- farin and phenytoin.

Question 35

Macrolides: Erythromycin

Answer 35

This is usually administered orally or i.v. by slow infusion. 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 infec- tions. 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.

Question 36

Aminoglycosides

Answer 36

They are valuable drugs for severe Gram negative infec- tions, usually given in combination with a β-lactamase antibiotic. The most commonly used are gentamicin, amikacin and tobramycin.

Question 37

Gentamicin

Answer 37

Gentamicin
This is usually given i.v. but can also be given i.m. It is active against coliforms, Ps. aeruginosa and staphylo- cocci. Streptococci and anaerobes are resistant.

Question 38

Amikacin

Answer 38

Amikacin

This is reserved for life-threatening infections with gentamicin-resistant organisms with proven amikacin sensitivity.

Question 39

Tobramycin

Answer 39

This drug is particularly useful in infections due to
Ps. aeruginosa.

The major side effects of aminoglycosides are oto- toxicity (vertigo or deafness) and nephrotoxicity. Therapeutic levels depend on renal function. Serum levels must be monitored. Accurate monitoring of levels is essential in patients with impaired renal func- tion and patients on long-term therapy.

Question 40

Quinolones

Answer 40

Ciprofloxacin
This is usually given orally or i.v. It is a broad spectrum antibiotic against Gram negative bacteria, including Ps. aeruginosa, and staphylococci. Anaerobes are resist- ant. Its uses in surgery include urinary tract infections, especially those that are catheter-related, prostatitis and skin and soft tissue infections with Ps. aeruginosa. It is also useful for chest infections, especially those due to Gram negative organisms. Most strains of MRSA and an increasing proportion of Gram negative organ- isms are now resistant. The side effects include nau- sea, diarrhoea and vomiting. CNS side effects include anxiety, nervousness, insomnia and rarely convulsions. Ciprofloxacin potentiates warfarin.

Question 41

Metronidazole

Answer 41

This is widely used in surgery both prophylactically and therapeutically. It may be given orally, i.v. or rectally. It is active against anaerobic bacteria, e.g. bacteroides and clostridia. It is also active against the protozoal organisms Entamoeba histolytica and Giardia lamblia. It is used for intraperitoneal sepsis and gynaecological sepsis.
It is used prophylactically in appendicitis against wound infection (usually given rectally) and in color- ectal surgery, where it is given i.v. with induction of anaesthesia. It is also administered for giardiasis, intestinal amoebiasis and amoebic liver abscess. The side effects include anorexia, a sore tongue and an unpleasant metallic taste. It potentiates warfarin.

Question 42

Tetracycline

Answer 42

This is of limited use in surgery. It may be used in chronic bronchitis, non-specific urethritis and atypical pneumonia.

Question 43

Fusidic acid

Answer 43

This is usually used for penicillin-resistant staphylo- coccal infections and staphylococcal osteomyelitis. Tissue concentrations are good. It may be adminis- tered orally or i.v. Resistance arises easily and pref- erably it should be used in combination with another anti-staphylococcal agent.

Question 44

Vancomycin

Answer 44

This may be given orally or i.v. It is active against staphylococci (including methicillin-resistant strains), streptococci, enterococci and clostridia. Its chief use is for severe infections. Recently its use has increased due to intraperitoneal administration in CAPD peri- tonitis. Side effects include phlebitis when given i.v. ototoxicity and nephrotoxicity. Serum levels must be monitored to control dosage.

Question 45

Teicoplanin

Answer 45

Teicoplanin is a bacteriocidal glycopeptide active against both aerobic and anaerobic Gram positive bac- teria. It is usually administered i.v. but may be given i.m. It is active against Staph. aureus and coagulase positive staphylococci (sensitive or resistant to methicil- lin), streptococci, enterococci, Listeria monocytogenes, micrococci and Gram positive anaerobes, including Clostridium difficile. Teicoplanin is chemically related to vancomycin, with similar activity and toxicity.

Question 46

Antibiotics in surgery

Answer 46

Antibiotics are never a substitute for sound surgical technique. Pus, dead tissue and slough need remov- ing. Antibiotics should be used carefully and only with positive indications. Prolonged or inappropriate use of antibiotics may encourage resistant strains of organisms to emerge. Except in straightforward cases, advice should be sought from a microbiologist.

Question 47

Selection of antibiotic for surgery

Answer 47

Selection of antibiotic
The decision to prescribe antibiotics is usually clinical and is based initially on a ‘best guess’ policy, i.e. based on experience of the particular condition, what the organism is likely to be, and to which antibiotic it is most likely to be sensitive.
The following sequence of events usually occurs in selection of an antibiotic:
1. A decision is made on clinical grounds that an infection exists.
2. Based on signs symptoms and clinical experience, a guess is made at the likely infecting organism.
3. The appropriate specimens are taken for microbiological examination, i.e. culture and sensitivity.
4. The cheapest, safest and most effective drug or combination of drugs effective against the suspected organism is given.
5. The clinical response to treatment is monitored.
6. The antibiotic treatment is altered if necessary
in response to laboratory reports of culture and sensitivity.

Question 48

Poor response of antibiotic

Answer 48

Occasionally the response of the infection to an apparently appropriate antibiotic is poor. Possible causes for this include:
• failure to drain pus, excise necrotic tissue, or remove foreign bodies
• failure of the drug to reach the tissues in therapeutic concentration, e.g. ischaemic limb
• the organism isolated is not the one responsible for the infection
• after prolonged antibiotic therapy, infection with new organisms develops
• inadequate dosage
• inappropriate route of administration

Question 49

Prophylactic antibiotics

Answer 49

Despite aseptic techniques, some operations carry a high risk of postoperative wound infection, bacteraemia, or septicaemia. Administration of antibiotics in the perioperative period will reduce the risks.

Indications for prophylactic antibiotics
• implantation of foreign bodies, e.g. cardiac prosthetic valves, artificial joints, prosthetic vascular grafts

• patients with pre-existing cardiac disease who are undergoing surgical procedures, including dental procedures, e.g. patients with mitral valve disease, as prophylaxis against subacute bacterial endocarditis
• amputation, especially for ischaemia or crush injuries where there is dead muscle. The risk of gas gangrene is high, especially in contaminated wounds.

Penicillin is the antibiotic of choice
• diabetics;
• immunosuppressed patients;
• organ transplantation;
• compound fractures and penetrating wounds;
• surgical incisions where there is a high risk of
bacterial contamination, i.e. clean contaminated wounds or frankly contaminated wounds
(e.g. bowel preparation for colonic surgery).

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 polish- ing), 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 (1h 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.

Question 50

Boils, styes and carbuncles

Answer 50

A boil (furuncle) is an infection of a hair follicle. A stye (hordeolum) is an infection of a hair follicle on the eye lid. A carbuncle is a group of boils interconnecting in the subcutaneous tissue by tracts. These infections are painful but not serious. Antibiotics are rarely indicated for boils and styes but may be appropriate for carbuncles. Infection is usually due to Staph. aureus, which is usu- ally an endogenous strain carried in the nose or on the skin. Boils may be recurrent, appearing in crops over several weeks or several months. They may be a presenting sign of diabetes. Antibiotic therapy is indicated only in certain cases: e.g. boils on the ‘dangerous area’ of the face where venous drainage is to the cavernous sinus and where cavernous sinus thrombosis may result; and also in the immunocompromised patient and diabetics.

Question 51

Erysipelas

Answer 51

This is a spreading infection of the skin due to Streptococcus pyogenes. It presents as a raised, red, indurated area of the skin which is sharply demarcated. The patient may present with high fever and appear toxic. It is a rare condition at the present time but responds well to penicillin.

Question 52

Cellulitis is a spreading infection of the subcutaneous tissues: Acute pyogenic cellulitis

Answer 52

This is due to Strep. pyogenes and presents as a red, painful swelling, usually of a limb, being commonly associated with lymphangitis and lymphadenitis. It is particularly likely to occur in the lymphoedematous limb. Treatment is with penicillin.

Question 53

Cellulitis is a spreading infection of the subcutaneous tissues: Anaerobic cellulitis

Answer 53

This is rare and is usually due to anaerobes, including clostridia, but more often is due to synergistic infec- tion with both aerobes and anaerobes. The causative organisms are usually a combination of anaerobes (bacteroides, clostridia, anaerobic cocci) and aer- obes (coliforms, Pseudomonas aeruginosa and Strep. pyogenes). Clinically, redness and oedema present around a wound (surgical or traumatic). This may progress in two ways, as follows.

Question 54

Cellulitis is a spreading infection of the subcutaneous tissues: Bacterial gangrene

Answer 54

Bacterial gangrene
The skin becomes purple and ischaemic and eventu- ally undergoes necrosis. Fournier’s gangrene of the scrotum is an example.

Question 55

Cellulitis is a spreading infection of the subcutaneous tissues: Necrotising fasciitis

Answer 55

In this condition the skin remains normal in the early stages whilst the infection spreads along fascial planes, causing extensive necrosis. Later the overlying skin becomes deprived of its blood supply, loses its sensa- tion and eventually becomes purple, black and under- goes necrosis. This is a life-threatening condition in which the patient is seriously ill with fever, toxaemia and, occasionally, septic shock. Wide excision of the area of necrosis and infection, together with treatment with appropriate antibiotics is indicated. The mortality rate is high.

Question 56

Lymphangitis and lymphadenitis

Answer 56

Lymphangitis is a non-suppurative infection of lymph- atic vessels that drain an area of cellulitis. Lymphadenitis is infection of the regional lymph nodes as a result of infection in the areas which they drain. It usually, but not always, results from cellulitis and lymphangitis. Occasionally the nodes suppurate and form an abscess. Lymphangitis produces red tender streaks along the line of lymphatics extending from the area of cellulitis towards the regional lymph nodes. Lymphadenitis is represented by enlarged, tender, regional lymph nodes. Occasionally the overlying skin is red and the glands may be fluctuant. Treatment of both lymphangitis and lymphadenitis depends upon isolation of the appropriate infecting organism.

Question 57

Gas gangrene

Answer 57

Gas gangrene is a rare disease in peace time but is closely associated with grossly contaminated wounds due to war injuries. However, there remains a problem in civilian surgical practice in that clostridial infection can occur after elective surgery especially on the gas- trointestinal tract (Clostridium perfringens is a normal bowel inhabitant), lower limb amputation, or vascular surgery on the ischaemic limb. In the case of trauma it is due to contamination of wounds by dirt and soil which contain clostridia derived from faeces. Infection is favoured by extensive wounds with the presence of necrotic tissue which provides an anaerobic environment for clostridia to proliferate.

An anaerobic environment initiates conversion of spores to vegetative, toxin- producing pathogens. Clostridia proliferate and pro- duce toxins that diffuse into the surrounding tissue. The toxins destroy the local microcirculation. This allows further invasion which can advance extremely rapidly. The α toxin of Clostridium perfringens kills muscle cells and destroys fat. Gas formation occurs with local crepitus. As the disease advances, toxins are released into the systemic circulation, causing the clinical features of pallor, restlessness, delirium, tachycardia, jaundice and ultimately septic shock and death. With gas gangrene the surface oedema, necrosis, and discol- oration of the skin are less extensive than the underlying myositis. Diagnosis is confirmed by examining a speci- men of exudate or tissue after Gram staining, when the typical Gram positive bacilli are seen; and by culture.

Question 58

Tetanus

Answer 58

This is a rare condition in the UK, because of wide- spread immunisation. It is caused by Clostridium tetani, an anaerobic Gram positive bacillus which pro- duces a neurotoxin. It is found in soil and faeces. The neurotoxin enters the peripheral nerves and travels to the spinal cord where it blocks inhibitory activity of spinal reflexes, resulting in the characteristic features of the disease. The disease follows the implantation of the spores into deep, devitalised tissues.

There is usually a history of a wound which may be as minor as the prick of a rose thorn. The incubation period is 1–30 days. Muscle spasm usually occurs first at the site of inoculation and is followed by trismus resulting in the typical risus sardonicus (lockjaw). Stiffness in the neck, back and abdomen follow, together with gen- eralised spasms which may cause asphyxia. The muscles remain in spasm between convulsions.

Opisthotonos (arching of the back and neck due to spasm) may occur. This stage is followed by convulsions which are extremely painful and during which the patient is conscious. Death may occur from asphyxia due to involvement of respiratory muscles or from inhalation of vomit with aspiration pneumonia. The diagnosis is usually clinical. Attempts at bacteriological confirma- tion often fail. Tetanus is rare in the UK because of an active immunisation programme in childhood with tetanus toxoid. All children should be immunized with three doses at monthly intervals. Booster doses should be given at entry to school and then on leaving school. All patients attending an Accident and Emergency department with new trauma, however mild, should have a booster unless they have received five doses pre- viously. Contaminated and penetrating wounds should be debrided and prophylactic penicillin administered. Human antitetanus immunoglobulin should be given for wounds contaminated with manure.

Question 59

Abscess

Answer 59

An abscess is a local collection of pus. Abscesses are walled off by a barrier of inflammatory reaction (pyo- genic membrane), and fibrosis occurs, ‘encapsulating’ the abscess. It is, therefore, impossible to treat abscesses satisfactorily with antibiotics alone. Surgical drainage is also necessary.

Without treatment abscesses tend to ‘point’ spontan- eously to the nearest epithelial surface: e.g. skin (boil), gut (pelvic abscess to rectum), and bronchus (lung abscess). Spontaneous drainage often leads to healing provided the initiating stimulus has been eliminated. If spontaneous drainage does not eliminate the initiating stimulus, a chronic abscess may form, resulting in a con- tinuously discharging sinus or abscess which intermit- tently develops, discharges and heals. A good example of this is a stitch abscess or a stitch sinus which does not heal until the stitch is removed.

Treatment of an abscess, inappropriately, with antibiotics alone, may actually halt the expansion of the abscess, giving rise to a ‘sterile’ abscess or antibioma.

Pyogenic abscesses are caused by a wide variety of bacteria and occur at many different sites (Table 7.2). They may be clinically obvious such as in the breast, perianal region, or axilla, or they may be cryptic or hidden, e.g. subphrenic or pelvic abscesses. Abscesses do not necessarily form at the site of primary infection but may form at a more distant site, e.g. pelvic or sub- phrenic abscesses after perforated appendicitis, due to tracking of infected material.

‘Metastatic’ abscesses may form as a result of hae- matogenous spread or ‘pyaemic’ spread of infected thrombi. Portal pyaemia following appendicitis may
result in liver abscesses, and infective endocarditis may result in cerebral abscesses.