Abstract
Bacterial cells are mostly studied during planktonic growth although in their natural habitats they are often found in communities such as biofilms with dramatically different physiological properties. We have examined another type of community namely cellular aggregates observed in strains of the human pathogen Staphylococcus aureus. By laser-diffraction particle–size analysis (LDA) we show, for strains forming visible aggregates, that the aggregation starts already in the early exponential growth phase and proceeds until post-exponential phase where more than 90% of the population is part of the aggregate community. Similar to some types of biofilm, the structural component of S. aureus aggregates is the polysaccharide intercellular adhesin (PIA). Importantly, PIA production correlates with the level of aggregation whether altered through mutations or exposure to sub-inhibitory concentrations of selected antibiotics. While some properties of aggregates resemble those of biofilms including increased mutation frequency and survival during antibiotic treatment, aggregated cells displayed higher metabolic activity than planktonic cells or cells in biofilm. Thus, our data indicate that the properties of cells in aggregates differ in some aspects from those in biofilms. It is generally accepted that the biofilm life style protects pathogens against antibiotics and the hostile environment of the host. We speculate that in aggregate communities S. aureus increases its tolerance to hazardous environments and that the combination of a biofilm-like environment with mobility has substantial practical and clinical importance.
Highlights
Many bacterial species can grow either in the form of dispersed single cells in liquid or as densely packed communities attached to solid surfaces
As cells present in biofilms are commonly protected against antibiotics and host defense molecules, biofilm formation has serious clinical consequences and is a significant contributor to the health care problems associated with S. aureus [4,5,6]
Staphylococcus Aureus Aggregates in Solution Some strains of S. aureus are capable of forming large planktonic aggregates that are visible to the naked eye when grown under standard laboratory conditions [11,12,31]
Summary
Many bacterial species can grow either in the form of dispersed single cells in liquid or as densely packed communities attached to solid surfaces. Staphylococcal biofilms contain several matrix components including extracellular DNA (eDNA), protein and polysaccharide [2,7,8]. In S. aureus, eDNA is released from dead cells by controlled cell lysis and the presence of eDNA is important in the very early establishment of the biofilm [9,10]. At this early stage, S. aureus cell surface proteins such as fibronectin and fibrinogen binding proteins and Protein A are contributing [11,12]. The extracellular polysaccharide poly-N-acetyl-1,6-glucosamine (PNAG) is often involved in biofilm formation and it is the most characterized component of the biofilm matrix. The ica operon is present in many clinical S. aureus strains and its expression has been shown to be strongly induced in a device-related animal model, underscoring the importance of biofilms during infection [16]
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