Previously reported and redesigned primer and probe assays were evaluated for the quantitative analysis of the fecal indicator bacterial groups, Enterococcus and Bacteroidetes with three real-time PCR instrument and reagent systems. The efficiency and sensitivity of the original assays varied between systems in analyses of DNA extracts from pure cultures of Enterococcus faecalis and Bacteroides fragilis, whereas the modified assays gave more consistent results. Distinctions between original and modified assays also occurred in analyses of known spike levels of E. faecalis and B. fragilis cells on filters with diverse surface water retentates. Percentages of samples causing PCR failures due to inhibition were lower using the modified assays. The accuracy and precision of spiked bacteria measurements were also generally higher, although mean measurements of both target organisms were still significantly different between systems (p < 0.05). The accuracy and precision of spiked bacteria measurements by both modified assays were further improved using a new sample matrix control spike consisting of cultured Lactococcus lactis cells and a reference assay for this organism. Corrections provided by the L. lactis assay eliminated significant differences in E. faecalis measurements between all three systems and between two of the three systems in B. fragilis measurements.