1 2 3 average F 2,4 TAPC[t] 18.6 17.3 18.1 18.0 3.43 tm[Φi] 17.1 17.4 16.8 17.1 P >0.1 OD[t] 17.9 17.9 17.7 17.8   Method τ (min) –MM       Exp. 1 2 3 average F2,4 TAPC[t] 52.7 50.1 51.9 51.6 0.886 tm[Φi] 50.8 59.9 52.1 54.3 P >>0.1 OD[t] 50.1 53.8 49.4 51.1   The agreement between the E. coli τ from TAPC and

microplate methods was somewhat unexpected inasmuch as solution agitation (i.e., oxygenation) of the media in each plate’s wells would be less than that for solution agitation in either normal or baffled flasks which were used for the TAPC comparisons. GF120918 order However, we found (Fig. 1A, open symbols) that [O2] levels in even highly agitated liquid E. coli cultures at 37°C dropped as much as 72% (LB, normal flask) with 200 RPM shaking while they were consuming approximately

4-6 × 10-18 moles O2 sec-1 CFU-1 (Fig. 1B). Even the baffled flask culture showed a drop in [O2] of 40-57%. Simultaneously, no cultures (Fig. 1A, closed symbols) showed any perturbations in τ (~ 18 min); the 23 min τ seen with bubbling is probably greater due to evaporative cooling of the medium. Due to differences in both solution mixing and surface area-to-volume ratio, the [O2] levels in microplate wells must be even lower than flask cultures at equivalent cell densities. Fig. 1 demonstrates that even at the lowest [O2], the rates of growth were unaffected. Clearly, being a facultative selleck screening library anaerobe,

E. coli is able to rapidly adjust to different levels of O2 with no apparent change in its specific growth rate, although the maximum cell density in stationary phase is usually Selleck PCI32765 greater in highly oxygenated samples GNE-0877 by up to an order of magnitude. Figure 1 Steady state O 2 ([O 2 ]: Fig 1A, open symbols), O 2 consumption rates (normalized to TAPC: Fig 1B) and E. coli cell growth (Fig 1A, closed symbols) as a function of growth time at 37°C in various media. Culture volume = 100 mL minimal defined medium (MM) or Luria-Bertani (LB) broth in a 250 mL normal or baffled Erlenmeyer flasks; 200 RPM agitation: squares = MM, normal flask; circles = LB, normal flask; triangles = LB, baffled flask; diamonds = LB, air bubbled in addition to shaking. Effect of Initial or Starting CFU Concentration on τ While performing studies related to comparing various assays for determining growth rate (Table 1), we noticed that our test organism, a nonpathogenic avian E. coli isolate, seemed to display uniform OD[t]-based τ values up to a threshold CI, at which point there was an obvious increase in the observed τ scatter (Fig. 2). The main graph in Fig. 2 represents 653 measurements of τ derived from OD[t] data using Eq. 1 (Methods Section) plotted as a function of CI (diluted from stationary phase cells). When CI > ca. 100 CFU mL-1, τ was narrowly Gaussian-distributed (i.e., a unimodal distribution) with a total spread of ca.