In 1987, Trypanosoma brucei was the firstparasite for which introduction of functionalplasmid DNA was reported1. Since then,this has been achieved for an increasingnumber of unicellular and multicellularparasites, resulting in a vast increase in ourknowledge about these organisms. Arethese parasites now ‘transformed’ or‘transfected’?

Since the seminal work of Avery andcolleagues, in 1944, on DNA as the carrierof genetic information2, the term‘transformation’ has been used inmicrobiology to describe the geneticmodification of a bacterium by uptake offree DNA3. Later, ‘transformation’ was alsoused when the delivery of plasmid DNAinto bacteria or other organisms bychemical or biophysical means(electroporation) was meant4.

Unfortunately, the term ‘transformation’was introduced in parallel by cell biologiststo describe the ability of higher eukaryoticcells to proliferate indefinitely in culture,grow on soft agar and cause tumors inanimals5. This would not have caused aproblem if the introduction of foreign DNAinto eukaryotic cells was not so long incoming. To prevent confusion, the term‘transfection’ (coined from transformationand infection), initially defined as the‘infection of cells by the isolated DNA from a virus’6, is now used. To cite the On-line Medical Dictionary(http://www.graylab.ac.uk/cgi–bin/omd):‘transfection is analogous to bacterialtransformation but in eukaryotes,transformation is used to describe thechanges in cultured cells caused by tumorviruses’.

To complicate matters, whereas‘transfection’ with a plasmid containing, forexample, large T antigen from SV40 will

result in a transformed phenotype7,‘transfection’ by introduction of a reportergene such as luciferase apparently does notchange the phenotype of a mammalian cell.Thus, while transfection can lead totransformation (although not a necessaryconsequence), transformation is notdependent on introduction of foreign DNAor a virus, and it can also arisespontaneously through mutation ofchromosomal DNA, eg. of the tumorsuppressor gene p53 (Ref. 8).

But why bother? Although at first sight, itmight seem to be only a semantic problem(if at all) and be confusing only for theuninitiated, this confusion is ratherwidespread, with some practicalconsequences:

Performing a recent Medline search with‘parasite’ and ‘transformation’ as the queryresulted in 1154 citations! Manyelectroporators and gene guns would haveto be sold in the past few years if all thesereferences really fulfilled the intention ofthis search, namely the introduction ofDNA into parasites. Even excludingTheileria, which actually transforms infectedbovine lymphocytes9, and adding ‘genetic’as a search term did not greatly influencethe result. However, using the word‘transfection’ instead reduced the outputdrastically (194 citations). But did I get nowwhat I wanted? Unfortunately not, becauseseveral ‘transformants’ did not now showup in the ‘transfectants’, although theyshould have done because they hadreceived foreign plasmid DNA! Andresearchers who ‘transform’ or ‘transfect’the very same parasite and use these termsin the same context in their paper arescarce (24 citations)! Significantly, relevantpapers I was looking for would have beenmissed if only one of the terms was used,

ie. a recent review entitled ‘Genetictransformation of parasitic Protozoa’10 wasnot in the ‘transfection’ group, and one ofthe first descriptions of Plasmodiumtransfection did (correctly!) not mentionthat they had transformed the parasite11!

For the sake of faster Medline searcheswith minimum effort, and because it issimply unequivocal, I recommend to call itwhat it is – transfection!

References1 Gibson, W.C. et al. (1987) Stable

introduction of exogenous DNA intoTrypanosoma brucei. EMBO J. 6, 2457–2461

2 Avery, O.T. et al. (1944) Studies on thechemical nature of the substance inducingtransformation of pneumococcal types,induction of transformation by adesoxyribonucleic acid fraction isolated fromPneumococcus type III. J. Exp. Med. 79, 137

3 Rieger, R. et al. (1991) Glossary of Genetics,Berlin, Springer

4 Maniatis, T. et al. (1982) Molecular Cloning,Cold Spring Harbor Laboratory

5 Fell, H. (1960) Experimental transformationof cells. Nature 185, 882–884

6 Földes, J. and Trautner, T.A. (1964) InfectiousDNA from a newly isolated B. subtilis phage.Z. Vererbungsl. 95, 57–65

7 Jha, K.K. et al. (1998) SV40-mediatedimmortalization. Exp. Cell Res. 245, 1–7

8 Wang, X.W. (1999) Role of p53 andapoptosis in carcinogenesis. Anticancer Res.19, 4759–4771

9 Dobbelaere, D. and Heussler, V. (1999)Transformation of leukocytes by Theileriaparva and T. annulata. Annu. Rev. Microbiol. 53,1–42

10 Kelly, J.M. (1997) Genetic transformation ofparasitic Protozoa. Adv. Parasitology 39,227–270

11 Goonewardene, R. et al. (1993) Transfectionof the malaria parasite and expression offirefly luciferase. Proc. Natl. Acad. Sci. U. S. A.90, 5234–5236

Frank SeeberFB Biologie/ParasitologiePhilipps-Universität MarburgMarburg, Germany

Letters

404 Parasitology Today, vol. 16, no. 9, 2000 0169-4758/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.

‘Transfection’ vs ‘Transformation’: Defining Terms

We read with interest the Letter by JuhaLaakkonen in Parasitology Today lamentingthe lack of information on the ecologicalimpact of fungal disease agents on wildlifepopulations1. The comment is timelyconsidering the very recent discovery of afungal pathogen that is causing massmortalities associated with populationdeclines in amphibians on two continents2.This previously unknown pathogen,Batrachochytrium dendrobatidis3, belongs to the phylum Chytridiomycota, a group ofzoosporic, non-hyphal fungi that consists largely of heterotrophs andsaprobes4. Chytrid parasites of plants,invertebrates, protists and anaerobic

chytrids involved in cellulose digestion inherbivores have been reported4,5, however B. dendrobatidis is the firstmember of the phylum known to parasitizevertebrates2. This chytrid infects thekeratinized epidermal cells of amphibians,leading to a thickening of the stratumcorneum (hyperkeratosis), and causingdeath by an, as yet, unknownmechanism2,3,6. While other fungal agentshave been reported to cause amphibianmortality7,8, chytridiomycosis is unique in itsrecent panzootic emergence and itsassociation with mass mortalities, declinesand local extinctions of multi-speciesassemblages9.

In the case of chytridiomycosis, some ofthe ‘intriguing biological characteristics’ thatfungi possess1 appear to explain theparticularly high impact of this disease inamphibians from montane rainforests. Thepreference of B. dendrobatidis for coolconditions in vitro may predispose montanehost species to increased impact. The abilityof B. dendrobatidis to grow and reproducesaprobically in the absence of an amphibianhost (ie. facultative parasitism) may explainits ability to drive local populations toextinction by reducing threshold density toeffectively zero9.

We agree that fungal parasites areunder-represented in studies of the ecologyof infectious diseases1. However, theirimpact can be significant: amphibianchytridiomycosis adds to a growing numberof emerging infectious diseases (EIDs)affecting wildlife populations10, which

More on the Ecological Impact of FungalInfections on Wildlife Populations