Medical Microbiology and Infection

Bacterial classification is important, revealing the identity of an organism so that its behaviour and likely response to treatment can be predicted. Bacterial structural components Bacterial cell walls are rigid and protect the organism from differences in osmotic tension between the cell and the environment. Gram-positive cell walls have a thick peptidoglycan layer and a […]

Innate immunity and normal flora (Medical Microbiology and Infection)

The innate immune system, which consists of the normal flora, physical barriers such as the skin, antibacterial proteins and phagocytic cells, is an important defence mechanism against infection. Many responses to ‘harm’ are detected by pattern recognition molecules such as the Toll-like receptors (TLRs), which trigger cascades that activate phagocytes and the immune response. For […]

Pathogenicity and pathogenesis of infectious disease

Definitions Humans encounter bacteria, viruses and parasites that do not cause disease. An infection occurs when microorganisms cause ill-health. Term Definition Pathogen An organism capable of causing disease Commensal An organism that is part of the normal flora Pathogenicity The ability to cause disease Virulence The ability to cause severe disease The capsule of Streptococcus […]

The laboratory investigation of infection

Specimens Any tissue or body fluid can be subjected to microbiological investigation with the aim of identifying the infecting pathogen and predicting response to therapy. To optimize the diagnostic benefit it is necessary to: • understand in which tissues/specimens the organism is to be found and when in the natural history of the infection; • […]

Antibacterial therapy (Medical Microbiology and Infection)

Principles of antibiotic therapy Antibiotics aim to kill organisms while causing no harm to the patient – this concept is known as selective toxicity. It is best achieved by inhibiting bacterial functions that are not present in human cells, for example the peptidoglycan of bacterial cells is inhibited by penicillin. The difference between the dose […]

Antibiotics in clinical use (Medical Microbiology and Infection)

Beta-lactam antibiotics Penicillins work by inhibiting peptidoglycan cross-linkage. Modifications to the penicillins have extended their antibacterial spectrum and improved absorption. Penicillins now include: • natural penicillins (e.g. benzylpenicillin, penicillin V); • penicillinase-resistant penicillin (e.g. flucloxacillin); • aminopenicillins (e.g. ampicillin-like agents); • expanded-spectrum penicillins (e.g. piperacillin); • penicillins combined with β-lactamase inhibitors (e.g. amoxicil-lin and clavulanate, […]

Resistance to antibacterial agents (Medical Microbiology and Infection)

Resistance occurs when a previously susceptible organism is no longer inhibited by an antibiotic at a concentration that can be safely achieved in clinical practice. Resistance can develop quickly because: • bacteria multiply rapidly; • mutations arise regularly; • segments of DNA can transfer by transformation; • genes can be transferred rapidly by bacteriophages, plasmids […]

Sources and transmission of infection

Sources of infection Infection is caused either by organisms from the host’s normal flora (endogenous infection) or by organisms transmitted from another source (exogenous infection). Endogenous infection The normal flora will only invade if circumstances permit, as in some of the following examples. • Following bowel perforation Enterobacteriaceae and non-spor-ing anaerobes such as Bacteroides fragilis […]

Principles of infection control

Identifying an outbreak An outbreak, whether in a hospital or the community, must first be recognized by clinical awareness and effective laboratory diagnosis; data must be managed centrally, a process usually described as surveillance. Screening and diagnostics It is necessary to make a laboratory diagnosis on patients with infection (e.g. to identify methicillin-resistant Staphylococcus aureus […]

Infection in the hospital environment

Infection is a major cause of mortality and morbidity in patients admitted to hospital. The most frequent types of infection are urinary tract, respiratory, wound, skin and soft-tissue infections, and septicaemia, which is often associated with vascular access. The environment Food supply Food is usually prepared centrally in the hospital kitchens. Patients are at risk […]