Table 5.

Summary of the factors affecting the association of bacteria with particles

Particle type	Physical
DLVO	Derjaguin, Landau, Verwey and Overbeck double layer theory There are attractive and repulsive electrostatic forces between bacteria and particle surfaces that are stratified into three zones; a near zone (“primary minimum”) within 1 nm of the particles surface, in which bacteria are attracted to the particle, then an electrostatic repulsion zone, then a “secondary minimum” attractive zone 5–10 nm from the particles surface. The strength of attraction and repulsion between the bacterium and particle is affected by the ionic strength of the matrix such that increased ionic strength increases the repulsion.
Temperature	Decreased temperature decreases the energy available for adsorption and increases the viscosity of the bacterial cell wall or capsular polymers which also decreases adhesion.
Water flow	Higher flow/velocity reduces the contact time between bacteria and particles and also increases hydraulic shear which can disrupt the first stage of binding under DLVO.
DOM	Dissolved organic matter (DOM) Organic matter attached to particle surfaces may increase bacterial adsorption if positively charged, but organic material in the water may compete with bacteria for adsorption sites. The type and concentration of DOM might also influence bacterial chemotaxis/biofilm formation.
Composition and size of particle(s)	The size, surface area, volume, and surface roughness can all influence the number of adsorption sites and rate of disassociation.

Particle type	Chemical
pH	The effect of pH on bacterial adsorption is related to the characteristics of the adsorbing surfaces (e.g., carboxyl and amino groups on bacterial surfaces) and the ionic strength of the matrix. Particle surfaces commonly have a negative electrostatic charge, as do bacteria at neutral pH. Bacterial adsorption to inorganic particles increases as their zeta potential decreases.
Hydrophobicity	Bacterial hydrophobicity and charge increase during exponential growth, and this promotes adhesion to particles.
Ions	The ionic strength of the matrix affects electrostatic interactions between pathogens and particles. Divalent cations (e.g., Ca2+, Mg2+, Cu2+, Zn2+) promote adsorption by acting as a bridge between negatively charged particles/bacteria, more so than monovalent cations (e.g., Na+). Anions do not affect adsorption.
Gouy–Chapman	The charge on the surfaces of particles or bacteria is neutralized by oppositely charged ions in the water. This causes formation of a Gouy–Chapman diffuse electric double layer. Bacterial-particle adsorption is affected by the thickness of this layer, which is a function of ionic strength.

Particle type	Microbiological
Cell surface	Flagella, fimbriae, and pili have the effect of increasing diameter and promote the breach of electrostatic barriers. Motility increases the likelihood of bacterial–particle contact, and can overcome electrostatic repulsion.
Bacterial size and shape	Smaller bacteria more likely to be lodged in crevices in particles.
Extracellular polymeric substances (EPS) and biofilm	Extracellular polymeric secretions are often polysaccharides with the potential for hydrogen bonding and dipole–dipole-type interactions, and these characteristics promote irreversible adhesion even in the absence of favorable DVLO association conditions. Rarely applicable to pathogenic bacteria in the environment which do not actively grow and produce EPS. The presence of EPS may affect the adsorption of pathogenic bacteria to particles.
Chemotaxis	Bacteria are attracted to many chemicals, and this may play a role in particle adsorption.
Bacterial concentration	Particle adsorption may be proportional to cell concentration. The numbers of pathogenic bacteria are in turn related to factors that affect survival, such as pH, temperature, nutrient availability, and predators.