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The microbiology of diabetic foot infection

Diabetic Foot, The, Winter, 2003

INTRODUCTION

A satellite symposium held at The Diabetic Foot journal conference, on 22 September 2003, in London, discussed the management and treatment of diabetic foot infection. The symposium was chaired by Professor Gary French (Consultant Microbiologist, Guy's and St Thomas' Hospital and King's College, London) and Ali Foster (Lead Clinical Specialist Podiatrist, King's College, London). The co-chairs introduced the four speakers whose topics ranged from the microbiology of diabetic foot infection to management of infection and treatment with antibiotics and surgery.

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'Fifteen to twenty percent of all patients with diabetes develop foot ulceration at some time,' said Professor Gary French, Consultant Microbiologist, Guy's and St Thomas' Hospital and King's College, London. 'That is a huge percentage and infections requiring intravenous antibiotics develop in about 10% of patients with diabetes per year,' he added.

Infections range from mild superficial sepsis to severe cellulitis, necrosis and gangrene. The progression from relatively mild infection to major sepsis may be rapid, and, in addition to skin and soft tissue infection, there is also the danger of development of osteomyelitis. For all these reasons, we need to take urgent action to prevent limb-threatening uncontrolled sepsis leading to amputation.

Microbiology

Professor French presented the microbiological point of view of the treatment of infection. He emphasised the importance of distinguishing between bacterial colonisation and true invasive bacteria. Ulcers are often superficially colonised with many microbes that do not need treatment. Spreading erythema, the production of pus and systemic symptoms are all evidence of the development of invasive infection that will require therapy.

Professor French outlined the basic principles of treatment, namely to search for and treat infection, identify and correct inadequate arterial blood supply and address the underlying disease and provide good wound care. He explained that any necrotic tissue should be debrided and any pus should be drained and sent for culture. Drainage and debridement is important as an essential part of therapy in severe sepsis. Professor French stressed the importance of sending specimens to the microbiology laboratory before starting antibiotics. Specimens should be sent for culture and treatment with antibiotics should be based upon best guess of the infecting organisms until the results are confirmed.

Organims on foot infections

The organisms that do occur on foot infections are generally: Staphylococcus aureus and Streptococcus pyogenes arising from the patient's own skin, anaerobes from skin/bowel, and enterococci and coliforms from bowel.

Among the gram positive aerobes staphylococci are the most prevalent, and the most prevalent of those is S aureus. The next most common are streptococci.

S aureus

S aureus is the most important true pathogen in skin infections in general, and probably in uncomplicated diabetic ulcer infection as well. In patients who have complicated or limb-threatening disease the infection is often polymicrobial and more resistant organisms come into play, especially after the patient has been treated with various antibiotics. About 90% of all S aureus organisms are resistant to penicillins. These can be treated with a pencillinase-stable penicillin, such as fiucloxacillin, or a [beta]-lactam [beta]-lactamase inhibitor combination, such as co-amoxiclav.

Methicillin resistant S aureus

Methicillin resistant S aureus (MRSA) has become endemic in UK hospitals and is an increasing cause of infection in diabetic foot ulcers. This resistance is mediated by alteration in the penicillin-binding target site. Penicillin, cephalosporins and similar [beta]-lactam drugs bind to sites in the bacterial cell wall, preventing it from being properly formed, and thus causing cell death. These organisms are methicillin resistant, but the same mechanism produces resistance to all the penicillins and cephalosporins. In addition, MRSA has a special ability to accumulate other resistance mechanisms against other antimicrobials.

There are about II different classes of antibiotic that are normally effective against S aureus, but now we are seeing strains of S aureus that are resistant to nearly all of them, and in a few dreadful cases resistant to them all. MRSA would therefore be better called 'multiply resistant S aureus' and are much more difficult to treat than sensitive strains. Furthermore, MRSA tend to be hospital pathogens that spread easily between patients and between hospitals.

Professor French then talked about how MRSA has become more common throughout Europe. Figure 1 shows the increasing incidence of specific epidemic MRSA up till 1997 in hospitals in England and Wales. The proportion of S aureus that are now resistant to methicillin ranges from about 25 to 45%.

[FIGURE 1 OMITTED]

Factors associated with MRSA and diabetic foot infections

Professor French addressed the risk factors associated with MRSA. Since colonisation precedes infection, contact with other patients with MRSA is a risk factor. Other risk factors include repeated hospitalisations, lengthy hospital stays, prior antimicrobial therapy and the presence of surgical wounds.

Health Care Industry

The microbiology of diabetic foot infection

Diabetic Foot, The, Winter, 2003

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