MICROBIOLOGY

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 capsule is usually not  evident. However, when nitroge

FUNGI AS PARASITES: The impact of fungi on humans is potentially considerable when one considers the possible targets. As  parasites fungi primarily infect plants, and the Irish famine of the 1850s in which the potato crop was  ruined by Phytopthera infestans represents an alarming example of the damaging effects fungal infection  has on foodstuffs. Two million people died during the five-year period in which the crops were lost.  Less dramatically, fungi also cause spoilage of pharmaceutical and cosmetic products, as well as  attacking the timbers of our homes.  Only a small proportion of fungi (<0.1 per cent) exist as parasites of humans and animals, fungi mostly  acting as saprophytes (organisms that absorb their organic nutrients from dead substrates). By definition,  then, saprophytic organisms are not parasitic because their nutritional sources are not alive; instead the  organisms are acting as decomposers and recyclers. As most fungal infections are acquired from

VIRUSES AND CANCER: It may surprise people to hear that 10–15 per cent of all cancers across the globe can be attributed to  viruses. This figure is likely to increase as research uncovers more viral genes within the tumour cells.  One small benefit that has come about from the pandemic of AIDS has been the discovery that a new  herpes virus, human herpes virus-8 (HHV-8) has been identified to play a role in the development of  Kaposi’s sarcoma. The importance of understanding the role of viruses in the aetiology of cancer cannot  be overemphasised. Studies of viruses have led to many of the critical discoveries in the workings of  eukaryote cell biology: gene mutations in bacteriophages, cellular oncogenes, tumour suppressor genes, bacterial restriction enzymes and viral reverse transcriptase  All of these have been uncovered through the study of viruses. The study of such fundamental  mechanisms of cell biology provide new targets for the design of novel anticancer and ant

HOW VIRUSES DEAL WITH HOST IMMUNE RESPONSES: The host immune response to viruses is both humoral and cell-mediated. Circulating antibody will  neutralise extracellular virus in the body fluids (including mucous membranes) and the cell-mediated  arm will attack cells that express viral proteins. Previous infection or vaccination will be the two  relevant factors that dictate whether a person is able to swiftly neutralise an infection or not. In light of  these protective mechanisms, viruses have developed strategies to try to minimise their contact with the  immune response of the host. Viruses have adopted various strategies to out-manoeuvre the host, in  particular to evade or suppress the immune response that attempts to develop antiviral products.  It is useful to consider the broader strategies before looking at cellular mechanisms. •  Evasion of immune response.Viruses can replicate in tissues that are relatively protected from  surveillance by immune cells (e.g. brain

PRODUCTION OF INTERFERON: It was demonstrated in the 1950s that the culture medium bathing virus infected cell lines could inhibit  the multiplication of viruses in separate flasks. The active substances, termed interferons, are cytokines  (as discussed above) and are produced within hours of virus infection to limit the spread of virus in the  host whilst specific immune responses are developing. Most cell types can synthesise interferon but they  can be grouped into three broad types according to the cell type that manufactures them.  •  IFN- (alpha): leucocyte interferon, IFN- (beta): fibroblast interferon,  •  IFN- (delta): immune interferon (activated T-lymphocytes and NK cells).  Double-stranded RNA is the most potent stimulus for the production and release of interferon alpha and  beta which act to prevent or limit the surrounding cells from becoming infected. Interferons are  prophylactic rather than curative. The actions of interferons are somewhat varied in that th