Oxygen tolerance of human intestinal anaerobes 1 ’ 2

In the development of techniques for the isolation and identification of anaerobic bacteria there have been two major concerns: the elimination of molecular oxygen from the culture environment and the establishment of a negative oxidation-reduction potential (Eh) in the culture medium. Some time ago the question arose regarding the relative importance of these two factors in the multiplication of anaerobic bacteria. Is oxygen itself inhibitory or does an adverse Eh in the medium interfere with the multiplication of anaerobic bacteria? Results of early studies were inconclusive. Some of the data indicated that oxygen was toxic for anaerobic bacteria and other data indicated that a positive Eh was the growth limiting factor (1). In many of these studies atmospheric oxygen was used to increase Eh precluding differentiation of the effects of the two factors in inhibiting multiplication of the anaerobes. In more recent studies where the effects of oxygen were distinguished from the effects of Eh, molecular oxygen and not adverse Eh was the critical factor responsible for growth inhibition (2, 3). In these studies, a positive Eh was maintained by the addition of potassium ferricyanide to the culture medium and a negative Eh was maintained by the addition of dithiothreitol. When the cultures were aerated none of the anaerobic organisms multiplied irrespective of the Eh value of the medium. In the absence of oxygen, on the other hand, the organisms multiplied even when the Eh was poised at an Eh from + 370 my to + 500 my with potassium ferricyanide. In related studies, Onderdonk et al. (4) found that Bacteroides fragilis ssp. fragilis maintained in a chemostat was sensitive to the effects of dissolved oxygen but not adverse Eh. In the absence of oxygen, there was no demonstrable change in viable cell counts from the steady state even when the Eh was adjusted to + 300 my by the addition of potassium ferricyanide. However, when oxygen was introduced into the system, the viable cell count decreased at a rate comparable to the theoretical washout rate for a static bacterial culture. All of these results indicate that molecular oxygen, rather than adverse Eh, per Se, is the factor limiting the multiplication of anaerobes. When anaerobic cultures were aerated, organisms displayed different degrees of tolerance to oxygen. In comparing three species of anaerobic intestinal bacteria, Walden and Hentges (3) reported that Peptococcus magnus was the least tolerant. After 2 hr aerobic incubation, no viable organisms could be detected. B. fragilis was intermediate in tolerance with no viable organisms