Simple method for correct enumeration of Staphylococcus aureus.

Area of Science:

• Microbiology
• Bacterial Physiology
• Biotechnology

Background:

• Optical density (OD) is a standard method for estimating microbial cell numbers in liquid cultures.
• This method assumes uniform cell size and even dispersion, which is often not the case for bacteria like Staphylococcus aureus.
• Staphylococcus aureus is known to form aggregates, potentially leading to inaccurate cell estimations.

Purpose of the Study:

• To investigate the impact of Staphylococcus aureus aggregation on standard cell enumeration methods.
• To develop and validate a simple, efficient procedure to accurately determine cell numbers in aggregating Staphylococcus aureus cultures.
• To assess the applicability of the developed method for both optical density and agar plate counting.

Main Methods:

• Standard optical density measurements were performed on Staphylococcus aureus liquid cultures.
• A novel sonication procedure was introduced prior to OD measurements.
• Agar plate counting was also employed to validate cell number estimations.

Main Results:

• Staphylococcus aureus clinical and laboratory isolates commonly exhibit aggregation in tryptic soy broth.
• Aggregation significantly biases standard optical density measurements for cell enumeration.
• The proposed sonication method effectively disperses cell aggregates, enabling accurate OD-based cell number determination.
• Sonication also improved the accuracy of agar plate counting for aggregating strains.

Conclusions:

• Bacterial aggregation in Staphylococcus aureus cultures necessitates pre-treatment for accurate cell quantification.
• A simple sonication technique provides a reliable method to overcome aggregation-induced biases in OD and plate counting.
• This sonication procedure ensures accurate cell number estimation for diverse Staphylococcus aureus strains and growth conditions.