LifeScale AST study on a Proteus mirabilis Isolate

Consider the case of Proteus mirabilis isolate exposed to amoxicillin/clavulonic acid. The bio-mass of the three inoculates is seen to increase, although the two exposed to antibiotics do so at a slower rate than the control. Turbidity measurements of these samples would correctly show an increased microbe content for all three samples and if the bio-mass alone were used to determine susceptibility then the sample appears resistant.

If we examine the mass data for individual microbes we see a different result. Notice that the mass of the individual microbes exposed to the antibiotic has continued to increase throughout the measurement period. Compare this to the mean mass of the microbes in the control sample. Following an initial “plumping up” of the microbes the mean mass of the microbes in the control sample remains almost constant. This is expected; microbes do not grow forever.

The difference between the two data sets is easily explained if we take in to account the morphology of the microbes. Using LifeScale’s built in video microscope it is possible to directly observe the sample as individual microbe mass is measured. Observing the samples after a period of two hours confirms morphology changes in the antibiotic exposed samples.

Proteus mirabilis microbes exposed to amoxicillin/clavulonic acid show the development of both filaments and spheroplasts. Thus the increase in total bio-mass in the presence of the antibiotic can be attributed to changing morphology and not to increasing microbe concentration. This can be confirmed by taking into account the concentration data for the same sample. The concentration of the two samples exposed to the antibiotic show a negligible increase in concentration relative to the control.

When all of the available data, mean microbe mass, total bio-mass and the microbe concentration are considered together with the video images, there is no doubt that the sample is susceptible to the antibiotic, even the lowest concentration of 8/4 ug/ml.

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