Detection of viruses in a patient's secretions or tissue provides direct evidence of current or ongoing infection (Table 48.1). This can be by:

• virus culture (also referred to as cell or tissue culture)

• electron microscopy – visualization of whole virus particles

• detection of viral antigens

• detection of viral genome (RNA or DNA) by molecular techniques.

This chapter will discuss the first three techniques; molecular diagnosis is discussed inChapter 49.

Virus or cell culture

Viruses, like bacteria, can be cultured in the laboratory. However, viruses are fastidious intracellular organisms and therefore living cells are required to grow viruses in the laboratory. Many cell lines have been developed to support the growth of different viruses. A single type of cell line is not adequate, as specific viruses need specific receptors on the cell surface to which they attach to gain entry into the cell and to initiate replication. The presence of specific cell receptors on the cell surface determines which viruses will be able to infect them, and this is called ‘viral cell tropism’. For this reason many cell lines have to be maintained in a diagnostic laboratory. Another problem in the laboratory is to maintain these living cells in culture long enough to allow sufficient virus growth.

A suspension of cells in growth medium (consists of a buffer plus calf serum to provide protein and amino acids, and antibiotics to prevent bacterial overgrowth) is put in glass or plastic tubes/flasks, the cells attach to the sides of the container and grow until they become confluent.