252 MANUAL OF MICROBIOLOGICAL METHODS 



Antibiotics as well as other chemicals have been used to inactivate or to 

 inhibit bacteria where these organisms contaminated a viral or rickettsial 

 suspension. Needless to say the chemical must be one which affects only 

 the bacteria adversely. Such a method will prepare the mixture satis- 

 factorily for some routes of inoculation such as into the respiratory tract 

 or intraabdominally. However, the use of an inoculum containing an 

 appreciable quantity of some antibiotics, such as penicillin or strepto- 

 mycin, may produce neurological signs and even death when inoculated 

 intracranially. The amount and choice of the antibiotic will depend on 

 the task it has to perform. Ordinarily 100-500 units of the antibiotic per 

 milliliter of inoculum are employed. Usually a short incubation period 

 is advisable to allow the antibiotic time to deal with the bacteria before 

 inoculation. The antibiotic may be injected into the abdominal cavity 

 of the animal prior to the intracranial inoculation and obviate toxic reac- 

 tions due to the antibiotic. 



While bacterial contamination may represent one of the more common 

 hazards to the virologist, viral contamination can be more insidious and 

 therefore more disastrous. As an example it has been demonstrated that 

 two different types of influenza virus can be propagated together over a 

 considerable period of time without either overcoming the other (Sugg, 

 1951). It takes little imagination to surmise the amount of trouble 

 inherent in such an experience as far as research or diagnostic work is 

 concerned. Viruses that grow under widely divergent conditions, differ 

 in susceptibility to antibiotics, or are not related antigenically usually are 

 separated with relative ease. Strains within a type could be impossible 

 to separate. A situation as awkward as the latter one should serve to 

 make the virologist ''virus-contamination-conscious." In addition to all 

 the usual procedures to avoid contamination it is advisable to work with 

 only one strain at a time and between times to institute decontamination 

 procedures. It takes little imagination to project mixed-strain antigen 

 to mixed-strain antibody and utter confusion. It is very necessary to 

 have pure virus antigen and antibody available to obviate the strain- 

 contamination dilemma. 



While the quantities of inoculum suggested in Chap. X usually suffice 

 for the virologist, the question of concentration of virus in the inoculum 

 requires special attention. The concentration must vary considerably 

 depending upon the results desired. For antibody production a maxi- 

 mum antigenic stimulus is commonly desired. If inactivated virus is 

 used or if the virus is inoculated by a route wherein propagation of the 

 virus is not anticipated, a high concentration of the virus antigen, but 

 short of a toxic dose, is desirable. This is usually necessary because of 

 the relatively small mass of viral antigen present in the inoculum. Con- 

 sidering the other extreme, a relatively small concentration of virus is 



