1. Introduction to microorganisms
Microorganisms are mostly harmless and non-pathogenic (non-disease causing), and indeed may be beneficial. It is estimated that a human body carries approximately 1014 cells, but only 10% of these are human in origin; the rest is microbial flora. There is not a consensus definition of a microbe but broadly speaking they are organisms which are not visible to the naked eye. Medical microbiology is the study of microscopic organisms and their effect on humans. Traditionally, slightly larger organisms that cause infectious disease, such as helminths, are usually included and this book will keep to that tradition. It encompasses their biology, diagnosis, treatment and prevention.
If morbidity and mortality from infection is to be reduced, a number of issues must be considered. The environment must be managed through public health measures to reduce the chances of contact with virulent microorganisms. In hospitals, this process is called ‘infection control’ and it includes steps to ensure that patients with hazardous agents do not disseminate them to others. Innate and specific immunity are clearly important in determining the outcome of contact with pathogenic organisms; we must understand how immunity works, what happens when it is disturbed through modern medical treatments and how it might be increased by methods such as immunization. Disease must be diagnosed quickly and accurately, either clinically or through laboratory methods, before it has spread to others or the individual is too ill to be saved. We must develop and apply high-quality, evidence-based treatments which include the prompt and appropriate use of drugs; the ideal antibiotic will kill the infecting microorganism but not the commensal bacterial flora or the patient, and yet will not lead to antibiotic resistance amongst virulent bacteria over time. Infections are extremely common, and it is vital that all medical doctors thoroughly understand basic microbiology if they are to prevent, diagnose and treat infections effectively.
Prokaryotes and eukaryotes
There is a vast array of agents that are capable of causing human disease (Fig. 3.1.1). A ‘family tree’ of all living organisms is shown in Fig. 3.1.2. This tree, with its three main branches, is very different from that suggested in the 1980s and has come about because of advances in molecular biology. The eukaryotic domain is a single group with almost unbelievable diversity, from single-celled amoebae through worms, fungi and plants right up to complex animals such as humans.