Of 153 clinical isolates of shigellae examined, 64.7% belonged to Shigella flexneri, 18.9% to Sh. sonnei, 11.8% to Sh. boydii and 4.6% to Sh. dysenteriae. Part of these isolates were resistant to sulfamethoxazole and streptomycin (88.2% each), ampicillin (66.7%), tetracycline (63.4%) and co-trimoxazole (43.1%), with levels of resistance (MIC50 and MIC90) being invariably high. Resistance to three or more drugs (multidrug resistance) was seen in 77.8% of the isolates. All the 25 strains examined for transfer of resistance contained R-plasmids, both autotransferable and non-autotransferable (mobilized by transfer factor X). The frequency of transfer of different r-determinants varied from 2.7 X 10(-8) to 1.4 X 10(-3).
During the second half of the nineteenth century, infectious diseases were a prominent cause of morbidity and mortality. Almost one-third of the infants in the United States and Western Europe were unable to reach their first birthday due to causes attributable to infectious diseases.
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