Basics in microbiology for the patient requiring intensive care

The major aim of the article is to clearly define the basic concepts of carriage, colonization and infection. The individuals defence system consisting of three potent lines of defence against carriage, colonization and infection is described in detail. Surveillance samples of throat and rectum are required to measure carriage defence and are distinguished from diagnostic samples including lower airway secretions and blood for the evaluation of the quality of the last two defence barriers against colonization and infection.Microbes and infections are classified using the concept of the carrier state. Low level pathogens are distinguished from high level pathogens and potentially pathogenic microorganisms. Low level pathogens including viridans streptococci, enterococci and coagulase negative staphylococci only cause morbidity. High level pathogens including Salmonella and potential pathogens such as Streptococcus pneumoniae, Staphylococcus aureus, Klebsiella, Acinetobacter baumannii and Pseudomonas aeruginosa causing pneumonia and/or septicaemia, cause attributable mortality. The concept of the carrier state allows the distinction of primary endogenous from secondary endogenous and exogenous infections. Seven recent studies demonstrate that the major infection problem is primary endogenous due to potential pathogens imported into the unit, varying between 50% and 75% in adult and paediatric units respectively. Nosocomial infections due to intensive care unit (ICU) microbes only include secondary endogenous and exogenous infections. Infection control targets the three types of infection mainly due to potential pathogens. To control primary endogenous infections parenteral antibiotics are administered immediately on admission. Eradication of digestive tract overgrowth by abnormal flora using enteral non-absorbable antibiotics controls secondary endogenous infections besides restoring systemic immunity. A high level of hygiene is required to prevent exogenous infection. Surveillance cultures are indispensable for the evaluation of the efficacy and compliance of this protocol, termed selective decontamination of the digestive tract (SDD) which has been shown to reduce morbidity and mortality by 65% and 20%, respectively.