Abstract:
The past century saw the discovery of antibiotics as an effective treatment for bacterial
infections. Life-threatening infections that were previously untreatable could now be
neutralized by antibiotics with few side effects. However, in recent years this once effective
treatment has become less efficient as the bacteria it once treated are becoming increasingly
resistant. Antibiotic resistance presents a major concern in human public health not just on a
national level, but potentially at a global level. The development of bacterial resistance to
antibiotics is a natural response to stress.
Plants have always been an important source of medicines and treatments. In cases where
bacteria have become resistant to antibiotics, there have been notable successes in
re-establishing antibiotic effectiveness when the antibiotics were used in combination with
antibacterial plant extracts. The aim of this study was to compare the metabolites of an
initially susceptible wild-type (WT) Staphylococcus with its mutated strain which had lost its
antibiotic susceptibility. The mutated antibiotic resistant strain was made by exposing the
susceptible WT strain to starvation stress. Metabolites from both strains were then extracted
in methanol and analysed by 1H-NMR. Multivariate analyses of the NMR spectra showed
significant differences in metabolites between the WT and resistant strains. LC-MS and GC-MS
analyses of methanolic bacterial extracts showed that staphyloxanthin, a carotenoid that aids
in pathogenesis and protects against oxidative stress was partially identified in low
concentration from the WT strain. This was confirmed by comparing mass spectra with data
on the NIST (National Institute of Standards and Technology) library database. The susceptible
WT strain produced two staphyloxanthin precursors, compounds very similar to the two long
chain parts of staphyloxanthin, while the resistant strain did not produce any detectable
staphyloxanthin or its precursors. The precursors detected by GC-MS analysis and determined
from the NIST Library were similar to squalene and tetradecanoic acid. These were the long
hydrocarbon chains on the staphyloxanthin structure. This comparative model of analysing a
WT and its consequently resistant strain could also be used as a platform to test the effectiveness of different plant treatments against bacterial defence mechanisms like staphyloxanthin. Focusing on the mechanisms of resistance without impeding bacterial growth might reduce the rate of adaptive resistance in-turn, thereby reducing the rate of antibiotic resistance during antibiotic therapy.
