March, 1958]
S.
N.
BAKSHI
5
Table I shows that the combination of oxytetracycline and polymyxin β
sulfate is bactericidal in as high dilution as 0.03 mg. and 10 units per ml. of serum
broth for all mastitis organisms used in this experiment. Among the coagulase
positive Staphylococci, strain 8F proved to be the most resistant and was not killed
by Dilution IV. The rest of the Staphylococci remained alive in Dilution V.
Strains 5F and 17F resisted Dilution V, strains 20/1, 21/4 and 42/1 Dilution VI and
21/2 Dilution VII. The single strain of Escherichia coli used in this experiment
grew from Dilution VII within 24 hours and growth also appeared from Dilution
VI after 48 hours.
The antibiotic was observed to have sedimented at the bottom of the serum
broth tubes and the supernatant broth did not show any visible turbidity even in
the dilutions from which the viable organisms were later recovered on blood agar
plates.
Growth appeared in all the control plates after 24 hours.
In blood-agar plates the results were different and rather reverse. In this case
also strain 8F was more resistant among the Staphylococci and grew in Dilution VII.
Dilution VIII was bactericidal for 7F and 33/1 but all other test organisms including
Escherichia coli grew in this dilution after 48 hours. Strains 5F and 17F of Strepto
cocci resisted comparatively higher concentration of antibiotics. The growth of 5F
appeared in Dilution VI after 24 hours and 17F grew after 48 hours. Strain 5F
had not responded to the treatment in vivo. Strain 20/1, 21/4 and 8F grew in
Dilution VII and 21/2 and 42/1 in Dilution VIII after 48 hours.
Strain 42/1, Str. uberis had resisted in vivo but did not show any higher resistance
in vitro to the action of antibiotics as compared with the other Streptococci which
had responded to the treatment.
A study of the results of both the methods suggested that the nature of growth
of bacteria may be one of the reasons of Staphylococci and E. coli showing com
paratively higher resistance in liquid medium. The organism with a tendency
to settle at the bottom in the liquid medium and leaving the supernatant fluid clear,
were more prone to the action of the antibiotics which sedimented at the bottom
of the tube than those which multiplied and were homogeneously distributed in the
medium giving it a uniform turbidity. When the antibiotic was homogeneously
mixed in the solid medium it had an equalised action on the bacterial growth which
was planted on its surface.
Further, the in vivo sensitivity of the organisms has shown a correlation with
the in vitro results in plate method, but not in the tube method.
Str. lactis showed difference in sensitivity to the antibiotic as compared with
Str. pyogenes and Str. agalactiae but no marked difference was observed from Str.
dysgalactiae and Str. uberis. Coagulase positive Staph. aureus exhibited enhanced
resistance against the antibiotic as compared to coagulase negative strains. The
results are suggestive of a comparatively better developed mechanism of resistance
among the pathogenic strains.
Barnes [1955] and Morse [1950] have given the inhibitory levels of Oxy
tetracycline ranging between 1 to 3 mg. per ml. for Streptococci and Staphylococci,
whereas, in this experiment the bactericidal level in serum broth was 0.03 mg. of