EFFECTS ON THE GROWTH OF MICROORGANISMS 971 



Bacteria, Fungi, and Yeast 



Koch originally claimed that Hg++ possesses the ability to kill various 

 bacteria and their spores, but it was soon shown by Geppert (1889) that 

 the proliferative activities of the treated bacteria can be restored by remov- 

 ing the Hg++ with sulfides, and that the action of the Hg++ is primarily 

 bacteriostatic, a concept confirmed many times and extended to other or- 

 ganisms. Dilution or washing is not sufficient to extract the Hg++ bound 

 to the cells (Chick, 1908), but sulfides and various thiols (Fildes, 1940) can 

 readily reverse the bacteriostatic action. With high enough concentrations 

 and prolonged exposure, of course, bacteria may be killed, especially at 

 elevated temperatures. Yeast cells incubated with 0.93 mM Hg++ are killed 

 progressively over an hour as determined by staining with methylene blue 

 or Congo red (Rahn and Barnes, 1933). One of the first effects noted at 

 minimal concentrations of the mercurials is a prolongation of the lag phase 

 of proliferation (Cook and Steel, 1959). Increasing the concentration of mer- 

 curial progressively delays and slows the growth, and eventually stops it, 

 at which point the cells can remain viable often for quite extended periods 

 of time, as exhibited by the renewal of proliferation when the mercurial is 

 removed with a thiol. 



The sensitivities of various microorganisms to the mercurials are shown 

 in Table 7-26, in which only a few of the reported results have been pre- 

 sented because the purpose is mainly to illustrate that proliferation is 

 usually depressed at fairly low concentrations, and that the data often vary 

 due to the conditions of the testing. One of the most important factors is 

 the medium used, inasmuch as most growth media contain many substances 

 capable of complexing with the mercurials and reducing their effective 

 concentration. Claus (1956) demonstrated that the minimal inhibitory con- 

 centration of Hg++ varies over a 10-fold range depending on whether pep- 

 tone media or simple nitrogen sources are supplied, and Cook and Steel 

 (1959) also presented evidence that the usual culture media exert a pro- 

 tective action. Another important factor is the temperature, increase in 

 temperature markedly enhancing the bacteriostatic effect (Cianci, 1940; 

 Cook and Steel, 1959), the Q^q being 3-4 (Chick, 1908). There is unquestion- 

 ably species variation in susceptibility but no correlation with bacterial 

 metabolic or growth habits has been made. It should be noted that very 

 low concentrations of the mercurials occasionally stimulate growth, this 

 being perhaps more true for fungi than bacteria. Robertson (1943) noted 

 that a mercurial used for preserving leather actually accelerated the growth 

 of certain fungi within a particular concentration range, and Converse and 

 Besemer (1959) reported that p-MB at 0.00028 mM stimulates the growth 

 of Coccidioides immitis, although spherulation is definitely inhibited. This 

 phenomenon would probably be seen more commonly if low concentrations 

 of the mercurials were more frequently tested, and clinically and commer- 



