The kinetics of reversible and irreversible attachment of bacteriophage T 1

Abstract

Solutions are presented for differential equations representing the kinetics of reversible and irreversible attachment of bacteriophage to bacteria according to two different hypotheses: reversible attachment is an intermediate state in the infectious process (sequential theory); reversible attachment is noninfectious and competes with irreversible, infectious attachment (competitive theory). Curves computed from the respective solutions are compared. Experimentally, total attachment and irreversible attachment kinetics were followed during the attachment of phage T1 to Escherichia coli B(P1), to which T1 attaches normally, but in which T1 fails to produce progeny because of prophage interference. Consistent with the sequential theory, a shoulder was found under suitable conditions in the irreversible attachment curve. Consistent with the competitive theory, all attachment curves followed long enough showed a marked decrease in attachment rates after most of the phage had attached. The results on this point were not explainable on the basis of heterogeneity in the phage population. The curves for reversible attachment were more complex than could be explained by either theory. Differential equations are presented corresponding to the hypothesis (modified sequential theory) that attachment is a sequential process, but that a certain proportion of reversible attachment sites do not allow any subsequent irreversible step. Analytical solutions for these equations were not obtained, but curves were computed from them by numerical methods. By a trial and error process, it was possible to obtain curves that agreed quite well with experimental results. In some cases, the best fit could be obtained only by making the additional assumption that the phage population contained a small proportion of slowly attaching phages. The transition from reversibly bound to irreversibly bound state was directly observed by attaching briefly, at high bacterial densities, at 0°C, followed by dilution into attachment medium at 37°C. A large majority of the phage rapidly became irreversibly attached under these conditions. Stirring did not affect the result. From these results, it was concluded that reversible attachment is an intermediate state in the infectious process.