What question did this study set out to answer?

The aim is to evaluate how single and combinations of bacterial cultures respond to antibiotic treatments.

May 20, 2026Open Access

Divergent analysis of single and polybacterial cultures against commonly applied antibiotics

Key Points

The aim is to evaluate how single and combinations of bacterial cultures respond to antibiotic treatments.
Assessed antibiotic susceptibility of single, double, triple, and tetra bacterial cultures of S. aureus, E. coli, Streptococcus agalactiae, and Klebsiella pneumoniae isolated from milk.
Tested against 8 antibiotics based on antibiogram guidelines, measured by zone of inhibition (ZOI).
Calculated positive and negative percentage changes to compare sensitivities between combined and single bacteria.
E+SA+ST+K combination showed the highest ZOI against aztreonam (44.00±5.29 mm).
E+SA+ST combination had significant ZOIs for penicillin (30.00±4.00 mm), azithromycin (39.67±0.58 mm), ceftriaxone (27.00±2.65 mm).
Klebsiella pneumoniae exhibited the highest negative percentage change, indicating greater sensitivity when in combinations rather than alone.

Abstract

Bacteria respond differently when combined with each other in a culture media, and their responses against any chemical insult are expected to be different, likewise. It is common practice to study antibiotic susceptibility of single bacteria, whereas infections are mostly poly-microbial and hence findings cannot be truly implementable. The current study was planned to asses antibiotic susceptibility responses of single, double, triple, and tetra bacterial cultures of S. aureus (SA) , E. coli (E), Streptococcus agalactiae (ST) , and Klebsiella pneumoniae (K) isolated from milk. The bacteria were tested against n=08 antibiotics following antibiogram guidelines, and responses were recorded as zone of inhibition (ZOI) among single bacterial culture and mixed bacterial culture. Positive percentage change (PPC) was calculated to declare that the combination of bacteria was more sensitive compared to single bacterial culture, while negative percentage change (NPC) was measured to declare that the combination of bacteria was more resistant compared to single bacterial culture against a particular antibiotic. In current study, highest ZOI was shown by E+SA+ST+K combination against aztreonam (44.00±5.29 mm) which was many folds higher than the individual cultures. In case of E+SA+ST combination, penicillin (30.00± 4.00 mm), azithromycin (39.67±0.58 mm), ceftriaxone (27.00±2.65 mm) showed significant higher ZOIs, while triple combination (E+ST+K) exhibited following zones against ceftazidime (20.33±0.58 mm) and tetracycline (31.33±2.31 mm). There was highest resistance found against lincomycin as most of combinations did not show any ZOI. There were higher ZOIs in case of single bacterial cultures in E. coli (E) and ST while combinations of bacteria in other cases showed variable responses. Bacterial responses were significantly variable in single and combinations against enrofloxacin and aztreonam but values of ZOIs remained higher than tetracycline. Klebsiella pneumoniae consistently showed pronounced NPC more than others indicating its bacterial combinations to be more sensitive than to single culture. The study thus concludes high unpredictable responses of bacterial combinations against antibiotics as a reflection of risk factor for antimicrobial resistance due to overlooking polymicrobial culture susceptibilities. GRAPHICAL ABSTRACT

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