How many antibiotics are produced by the genus Streptomyces?

Milind G. Watve, Rashmi Tickoo, Maithili M. Jog, Bhalachandra D. Bhole

Streptomyces is the largest antibiotic-producing genus in the microbial world discovered so far. The number of antimicrobial compounds reported from the species of this genus per year increased almost exponentially for about two decades, followed by a steady rise to reach a peak in the 1970s, and with a substantial decline in the late 1980s and 1990s. The cumulative number shows a sigmoid curve that is much flatter than what a logistic equation would predict. We attempted to fit a mathematical model to this curve in order to estimate the number of undiscovered antimicrobials from this genus as well as to predict the trends in the near future. A model assuming that the screening efforts are encouraged by a previous year's success and that the probability of finding a new antibiotic is a function of the fraction of antibiotics undiscovered so far offered a good fit after optimizing parameters. The model estimated the total number of antimicrobial compounds that this genus is capable of producing to be of the order of a 100,000 - a tiny fraction of which has been unearthed so far. The decline in the slope appeared to be due to a decline in screening efforts rather than an exhaustion of compounds. Left to itself, the slope will become zero in the next one or two decades, but if the screening efforts are maintained constant, the rate of discovery of new compounds will not decline for several decades to come.