Bacterial Population Trends (image) Washington University School of Medicine Share Print E-Mail Caption School of Medicine researchers found that differing ratios of three major classes of bacteria -- Bacilli, Gammaproteobacteria and Clostridia -- colonized the guts of premature infants in sequence. Credit Washington University Usage Restrictions Please do not use until embargo lifts 3 p.m. ET Monday, Aug. 11, 2014. Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.