MICROBIOLOGY

THE VALIDITY OF  Ro: One of the values of Ro  lies in what its composition tells us. The characteristics that comprise Ro must be  essential and important for the organism, therefore reduce any of these values and the efficiency of  spread will be impaired. If any of the parameters are reduced to zero, in theory the epidemic will be  halted.  If   then the number of susceptibles (S), the efficiency of transmission (β) and infectious period (D)  will all be suitable targets for control of the epidemic. The easiest target will often be to reduce the   n umber of susceptibles through vaccination.  As a marker of the ability of an organism to spread, Ro  gives an indication of the propensity of an  organism to invade a population (cause an epidemic). Organisms of high Ro  will spread very efficiently  and infect the very young. Ro  can be used to predict the levels of vaccination needed to eradicate  inf...

This Post  considers some of the options available to help to control the impact of infectious diseases  on populations. The study of diseases within populations is called epidemiology. For microbiologists,  infectious disease epidemiology is an increasingly important area of study as infectious diseases do not  appear to be diminishing in number. The increased mobility of people across the globe has meant that  infectious diseases can be acquired in one part of the world and within hours transported to another. The  movement of micro-organisms will be the same. To keep an eye on the changes in distribution and  numbers of infectious diseases, national centres are required. Within the USA this is based at the Center  for Disease Control (CDC), Atlanta and in the UK, the Central Public Health Laboratory, Colindale,  London. This overview of national trends is part of the surveillance of infectious disease. The reason  that certa...

FUNGI AS PARASITES(CONTD..) ADAPTATION TO INCREASED TEMPERATURES (THERMAL TOLERANCE):  The temperature difference between the soil and warm-blooded animals is significant in light of the serious penalties incurred on enzyme function as the temperature increases. Those fungi that cause infections of the skin (dermatophytes) are restricted to temperatures cooler than 37°C, and may illustrate the problems of adapting to wider temperatures. For those fungi that do tolerate body temperatures, the shift in temperature is used as a signal for gene expression (a feature observed in pathogenic bacteria). ANATOMICAL STRUCTURES: The ability of certain fungi to use specialised structures to penetrate the host tissues is widespread amongst plant pathogens and also occurs in human fungal pathogens. The ability to switch between hyphal and yeast forms (dimorphism) is commonly observed when infecting animal hosts in the course of an infection. Candida develop hyphae (pseudohyphae) when...

CANDIDA ALBICANS : There are greater than 100 species of Candida but only C. albicans is regularly isolated from humans,  either as a member of the normal flora or from ill patients. Infections by C. albicans have increased in  incidence over the last 20 years as more and more medical advances provide opportunities for the  organism to colonise ill patients such that C. albicans is now the most common fungal infection in  humans. The reasons must lie partly with the fact that C. albicans is readily isolated as a member of the  normal flora of humans (mouth, gut and genitourinary tract) and any reduction in colonisation resistance  by antibiotics will permit a swift increase in numbers of Candida. Candida albicans causes localised  infections such as vaginal thrush which occur in otherwise healthy people, but is also an important cause  of systemic disease in hospitalised patients. The risk of colonisation by an endogenous organism i...

FUNGAL PARASITES(CONTINUED) A) CRYPTOCOCCUS NEOFORMANS: Cryptococcus neoformans is a yeast that has become a frequent invader of patients with impaired cell mediated immunity, whereas clinical disease is rare in healthy people. In such patients the yeast often  invades the central nervous system and is particularly frequent in causing meningitis, although  pneumonia is not uncommon. The organism can be recovered from soil throughout the world, thus  differing from the primary pathogens described above which have a restricted geographical location.  Cryptococci will favour nitrogen-rich soil, typically achieved through bird droppings. There are  approximately twenty species of cryptococci but only   C. neoformans   causes disease in humans.  Unusual amongst fungal pathogens is the production of a polysaccharide capsule by  C. neoformans . All  cryptococci can produce polysaccharide capsules but, when growing in soil, the c...