Date of Award


Degree Type


Degree Name

Master of Science in Microbiology




Sediment populations were tested for metal resistance by the spread plate method on Trypticase Soy Agar (TSA) amended with 50 ug/ml of Hg, Cd, Pb, Sn, or Zn. Those metals that were found in high concentration in Pawtuxet River sediment (i.e., Pb, Sn, and Zn) were less inhibitory to the sediment population than either Hg or Cd, present only in low amounts. Metal content of the Pawtuxet River sediment was determined by atomic absorbtion spectrophotometry. Several sediment isolates were selected and resistance patterns to the five metals were determined by the replicate method. A wide range of resistance was found to exist amoung the isolates. Resistance levels of these isolates were approximated by a gradient plate study, the metal concentration ranging from 0-200 ug/g. These tolerance levels were further pinpointed using metal amended Trypticase Soy Broth (TSB) to determine the minimal inhibitory concentration (MIC). The isolates were more resistant to those metals (i.e., Pb, Sn, Zn) that were present in the sediment from which the organisms were isolated.

Unpolluted sediment was subjected to pollution level amounts of a metal mixture containing Hg, Cd, Pb, Sn, and Zn, and Pb alone to determine the response of the microbial flora to the metals. Population levels in the test reactors initially decreased as compared to the nonpolluted control, but recovered later to levels equivalent to or greater than the control, and resulted in the selection of metal resistant predominant colony types. A cross tolerance study of predominant colony types demonstrated that Pb alone was not inhibitory, but that the metal mixture was toxic. Colony types selected by the metal mixture or Pb were generally homogeneous.

Plasmid screening of selected sediment isolates Flavobacterium PWX2 and Aeromonas PWX7 was accomplished via agarose gel electrophoresis to determine the relationship of plasmids to metal and/or multiple metal resistance. Flayobacterium PWX2, resistant to each metal (Hg, Cd, Pb, Sn, Zn) contained at least five plasmids; the plasmid pattern did not vary when the isolate was grown in each metal. Aeromonas PWX7 contained two or more plasmids, but yielded two distinct plasmid profiles: two plasmids were present when grown in Hg, while only one (the larger of the two found with Hg) was present when the isolate was grown in the presence of Pb or Sn.