Effect of Sodium Bicarbonate Concentration on Plaque Formation of Virulent and Attenuated Polioviruses

Summary The efficiency of plating (e.o.p.) of poliovirus under agar has been studied by the bottle culture technique. Plaque formation of attenuated strains under agar was usually restricted or delayed at low concentrations of bicarbonate (d marker in contrast to the d+ marker of wild virulent strains). Varying the bicarbonate concentration from 0.11 to 0.45 g % in liquid medium, did not significantly affect the adsorption and multiplication of attenuated or virulent strains. Strains passaged in monkey spinal cord retained the neuropathogenic character of the parent line. When the original inoculum was a virulent virus, the strains on further passage in tissue culture yielded d+ viruses. Similarly, when the parent virus was attenuated, the tissue culture progeny were of the d type. However, 2 exceptions were encountered where d+ progeny were produced, but they still behaved as attenuated strains for they were almost free of intracerebral neuropathogenicity for the monkey at the 105 PFU level. A number of newly isolated strains have been studied, using virus as present in human stools and in tissue culture progeny. In general, epidemiologically paralytogenic strains had a high e.o.p. in low bicarbonate medium, the property of a d+ virus. Strains which were epidemiologically mild, or which were recovered from normal children in interepidemic periods, fell into both the d and d+ categories. These d+ strains proved to be neuropathogenic for the monkey, sometimes demonstrable only by histological examination of the spinal cord. The d strains in this group failed to bring on any apparent disease in the test monkeys, but they did produce lesions in the spinal cord of a high proportion of the animals. When an attenuated d strain was fed to human beings (6, 7) this was followed usually by the excretion of d viruses, but in certain individuals by the excretion of d+ viruses. However, the d+ mutants were still of low neuropathogenicity for the monkey, suggesting that among the attenuated polioviruses any back mutation to virulence proceeds in small steps.