The article discussing factors affecting plaque formation by bacteriophages "How do you get bacteriophages to form plaques?" has been updated and is available at:  http://www.dairyscience.info/index.php/enumeration-of-lactococcal-bacteriophages/factors-affecting-plaque-formation.html - http://www.dairyscience.info/index.php/enumeration-of-lactococcal-bacteriophages/factors-affecting-plaque-formation.html .

Abstract

Current data indicate that some 10³¹ bacteriophages exist globally, including about 10⁸ genotypes. Some phages form very tiny or micro plaques. These can sometimes be so small that it is almost impossible to see them. Frequently 'new' phages can be observed using e.g. electron microscopy under conditions where there is strong evidence of a potential host yet it can be very time consuming or in some instances not possible to get the phage to form plaques. Less than 1% of the phages observed using microscopy have ever been grown in culture, this is sometimes called "the great plaque count anomaly".

The conditions required to get a newly isolated phage to form plaques have been reviewed. The importance of testing both logarithmic and stationary phage cells, a range of incubation temperatures, replacing agar with agarose, using low strength agarose top agar overlays in initial experiments, media supplemented with Ca⁺⁺ and Mg⁺⁺ that do not contain cation chelators, modifications to the double agar assay method including a) changes to the initial assay step so that phage adsorption takes place at ambient temperature  and b) the use of antibiotics and other activators of the host cell’s SOS system are discussed.