MICROBIOLOGY

PROTECTION AGAINST PATHOGENS: The normal flora offers some protection against invading pathogens. The pattern resembles the problems  of getting seeds to root in an established field of grain. The space is limited, the water and soil nutrients  are all being taken by established root systems. This protection, when applied to the intestine, is called  colonisation resistance, although the principle applies to any situation on the human body where the  normal flora limits the ability of an invading organism to gain a foothold. The efficacy of this protection  is highlighted by the disease pseudomembranous colitis(PMC). Patients with PMC develop severe  diarrhoea and have a characteristic membrane over the surface of the colon visualised by  sigmoidoscopy. PMC is caused by the multiplication and colonisation in the colon by the anaerobic  Gram positive rod Clostridium difficile . When the normal intestinal flora is disrupted by treatment with  poorly absorbed antibiotics, pat

THE NUMBER OF INFECTIOUS PARTICLES SHED: Evolutionary success means that the organism continues to propagate. As with the number of offspring  produced by all animals, the numbers produced depend on the likelihood that they will survive for  sufficient time to produce new offspring. With microbes, the more organisms that are shed, the greater  the chances of infecting a new host. Sexually transmitted infections are organisms that do not tolerate  drying and the route of transmission is quite demonstrably one in which the organisms are transmitted  without any time spent outside of the body. Correspondingly, there will be less need to release as many  organisms as would those organisms that risk desiccation during transmission. The stability of the     infective particle is thus another parameter of relevance. STABILITY OF THE MICRO-ORGANISM IN BETWEEN HOSTS: Airborne transmission runs both the risk of the organisms drying out and failing to not encounter a new  host whilst

MODE OF TRANSMISSION: The ability to reproduce will be the ultimate driving force for all organisms. For bacteria that infect  humans there are several routes that can be employed to infect new hosts. The various means by which a  micro-organism may be transmitted from the source to the new host.Note that  some organisms can be spread  by more than one mode of transmission. Measles virus is likely to be spread by airborne and direct  contact. The number of times people touch their faces with their hands is enough to almost guarantee  that virus in the nose or saliva will be transferred to the hands. You will notice other flaws in the  scheme. The table distinguishes between sexually transmitted infections and those obtained by direct  contact. It could be argued that they are the same mode of transmission. The example of dermatophytes  as directly transferred infections also needs qualifying. In the latter cases the fungi are often transmitted  through shared, inanimate obj

HUMAN INFECTIONS: The history of mankind is regularly punctuated with adverse dealings with microbes, both good and bad:  the great famine in Ireland; the consequences of smallpox on the native peoples of North and South  America; the influenza pandemics and the march of AIDS across the world. To help explain the variety  of different human infections, an understanding of their epidemiology is essential. Most people learn to avoid dangerous situations. Most try to avoid sitting next to someone sneezing and  blowing their nose. This behaviour is based on the recognition that germs are present. All infections  arise as organisms are transferred from a reservoir to a source and then transmitted to a susceptible  host. The transmission of microbes between humans is critically important in the successful spread of  the organism within a population. The often-called chain of infection is as follows:                                  Reservoir: Source: Mode of transmission: Host The rese

INTRACELLULAR PARASITES: The genera Chlamydia and Rickettsia both contain species that cause human disease. Chlamydia  trachomatis  causes trachoma and   Rickettsia prowazekii  causes typhus fever. Studies of the genomes of  these intracellular bacteria have yielded interesting patterns. By utilising host nutrients, these organisms  have lost the corresponding biosynthetic pathways; the amino acid synthesis pathways are absent with  the organisms choosing to utilise those of the host cell. Similarly the enzymes for synthesis of purines  and pyrimidines are largely absent. Energy requirements of these bacteria are still paramount but they  have lost the need for anaerobic glycolytic pathways as they now live in an aerobic environment. The  reduction in requirement for such enzymes results in the reduction in size of the genome of the bacteria.  The striking feature of these bacteria is the small genome size, around 1 megabase (1 Mb), roughly  equivalent to 800–900 genes, c