(a) The fluctuation test. Small numbers of phage‐sensitive Escherichia coli are taken from one large culture and used to set up seven smaller cultures. Cells from six of these subcultures are placed on agar plates in the presence of an excess number of phage. Meanwhile, the seventh subculture is divided into six fractions each of which are also placed on a phage‐containing plate. The numbers of phage‐resistant bacterial colonies from the six independent subcultures is quite variable. The numbers for the six samples which all came from the same subculture are much more consistent. (b) The origin of ‘jackpots’. Bacteria reproduce by dividing in two. Any mutation (grey) which occurs in one of the first few cells to appear in the culture will be present in all of its many descendants (left panel). A cell ‘born’ late in the culture period will leave few descendants carrying the mutation (right panel).

A bacterial conjugation experiment. The strain in tube A requires tryptophan for growth (W⁻), while that in tube B requires proline (P⁻). Therefore, neither will grow on minimal medium. However, the W⁻ strain is an Hfr strain, so it transfers part of its chromosome to the P⁻ strain when the two are mixed together. Some of the progeny are now both W⁺ and P⁺, so they are able to grow on minimal medium.

A bacterial transduction experiment. The strain in tube A is a proline prototroph, shown by growth on a medium lacking proline. That in tube E is a proline auxotroph, which shows no growth on proline‐deficient medium. Bacteria from A are mixed with a transducing phage (from tube B). Following infection and lysis in tube C, the new phage particles (without bacteria) are mixed in tube D with the auxotrophic strain from tube E. When the surviving bacteria are plated on proline‐deficient media, colonies appear. The phage has transferred the genetic information for proline synthesis from the prototroph to the auxotroph.