Infection can affect any tissue of the body, producing cell damage and inflammatory reactions. Viruses are generally too small to be seen in the light microscope, but their presence can often be inferred by the changes they produce in tissue, even if their identity requires confirmation by immunohistochemistry, serology or molecular biology. Bacteria can be seen in the light microscope using high magnification objective lenses; however the numbers of bacteria that are present in a tissue can be highly variable even in one disease. A classic example of this variability is leprosy, where there may be very large numbers of bacteria in the skin (lepromatous leprosy), or very few (tuberculoid leprosy). Distinguishing the type of bacteria in a thin section of a lesion generally requires specialised histological stains, although the morphology of the bacteria may also be informative (Figure 2). As with viral infection, the histological findings are an adjunct to serology and microbiology in producing a diagnosis.
(a) What stain could you use to identify M. tuberculosis in a section of lung? (b) What stain could you use to identify N. gonhorrea in a urethral smear?
(a) Ziehl Nielsen stains mycobacteria red; their identification is aided by the bacterial morphology - mycobacteria are rod-shaped. (b) Gram stain can help distinguish Neisseria, which are gram-negative streptococci from other streptococci and staphylococci which might also be found in the specimen.
Identification of parasites is often difficult by serological methods; however, the appearance of parasite-infected cells (e.g. malaria) or the parasites themselves is absolutely characteristic of the particular infection (Figure 3). Consequently, diagnosis of parasitic infections relies substantially on the initial histological or haematological findings.