Recombination in Bacteria

Abstract

In the late 1930s, no definite sexual differentiation of bacterial cells had been observed, no nuclei had been detected, and the chromosome had not been stained. However, the advent of selective techniques using nutritional mutants enabled a screen to be made for rare recombinant colonies. In addition, improvements in cytological techniques revealed the presence of darkly staining bodies (Robinow, 1944). Lederberg and Tatum (1946) devised experiments in which two parental bacterial strains of Escherichia coli were each genetically labelled with three different nutritional mutations, i.e. abcd ⁺ e ⁺ f ⁺ × a ⁺ b ⁺ c ⁺ def. Washed suspensions of the two types were plated on minimal medium and a ⁺ b ⁺ c ⁺ d ⁺ e ⁺ f ⁺ colonies were detected at a frequency of about 1 in 10⁶ bacteria plated. Reversion at three loci simultaneously could be ruled out as very improbable, and cross-feeding was ruled out by showing that the colonies could be cloned on to fresh medium. At first, transformation was thought to be the explanation of these results, but this was eliminated by showing that cell-free extracts or filtrates could not substitute for direct cell-to-cell contact. Initially it was assumed that the recombination shown by these experiments resulted from processes already discovered, such as fusion of nuclei, followed by meiosis or some other chromosomal segregation.