among the organisms that lead to opportunistic infections in humans, Candida albicans, a yeast-like organism, is the most common fungus. Oppor-tunistic infections are caused by organisms that live very close to humans and become active only if they detect weakness in the body such as lowering of the immune surveillance. The infections caused by Candida range from mild-skin infections to systemic infections that may be life threatening.
The yeast-like budding growth of Candida , sometimes changes to a filamented form that has outgrowing fibres known as 'hyphae'. The hyphae can penetrate tissues more easily than the budding yeast form. Such changes are considered to be a sign of disease. A study conducted by Ekkerhard Leberer and colleagues at the National Research Council in Quebec, Canada, explains the process of changes ( Current Biology , Vol 7, No 8).
A group of enzymes known as serine/threonine protein kinases are responsible for changes in the form of many organisms. Two enzymes, s te20p and c la4p, are involved in the transition from budding to filamentous growth in the common baker's yeast.
The researchers planned to search for a Candida equivalent to c la4p or c a c la4p, and understand its role in the development of virulence in the fungus. They used baker's yeast strain that lacked genes for both s te20p and c la4p. The absence of c la4p prevents normal cell division in the yeast whereas the absence of s te20p causes a defect in cell-to-cell signalling. Various fragments of Candida dna were introduced into the yeasts to find out whether the defects might be compensated for. A dna fragment was found that restored the ability of cells to divide but they were still unable to signal to each other. The sequence of dna within this fragment could code for a protein resembling serine/threonine protein kinases and was tentatively identified as the sought-after gene.
To investigate the function of c a c la4p, the researchers used a technique called homologous recombination. The growth of Candida cells was unaffected when one or both copies of the c a c la4p gene were removed. However, when both copies were missing, cell division was odd and resembled that found in baker's yeast after deletion of c la4p. Interestingly, these cells retained a yeast-like form and were unable to switch to the hyphal form.
Mutant Candida albicans cells were injected into mice veins and the animals monitored for fungal infection of the kidneys and survival. The normal (wild-type) fungus caused rapid death, and the mortality rate remained high when a strain deleted for a single copy of the c a c la4p genes was used. However, all mice survived when both copies of c a c la4p genes were deleted. The effects on survival went hand in hand with the ability of Candida cells to produce fungal infections in the kidney. And in all cases, the loss of infectivity or virulence caused by the lack of c a c la4p function could be reversed by restoring the function through a fragment of dna that carried the wild-type c a c la4p gene.
The results show a gene of known function in Candida albicans in both the yeast-to-hyphal transition and in virulence, but not in other 'housekeeping' functions (as judged by the lack of effect on growth rates). The study provides a strong base to use c a c la4p or other regulators of hyphal formation as potential targets in the fight against candidiasis.