Resistance to Antimicrobics

Biofilms possess a remarkable resistance to antibiotics sometimes more than 1000 times that of planktonic cells. Although the cause of this resistance is still under intensive investigation, several possible mechanisms have been advanced.

Failure of the Antimicrobic to Penetrate the Biofilm Matrix

In the early history of biofilm research this was a favored hypothesis. However, research by a number of investigators showed that in most cases, the antibiotic readily penetrates to the substrate surface. Except in specific circumstances (see next section) lack of penetration is not considered a significant contributor to antimicrobic resistance.

Degradation of the Agent

The antibiotic or disinfectant may be used up as it penetrates the biofilm. The degradation of penicillin by the enzyme penicillinase or the decomposition of hydrogen peroxide by catalase are examples.

Metabolic Heterogenity

A biofilm contains a vast array of different metabolic niches which vary in oxygen concentration, nutrient and ionic concentrations as well as concentrations of waste materials. Cells within the biofilm matrix vary in growth rate from actively growing to essential dormancy. Aerobes are active at the margins of the microcolony while anaerobes and facultative organisms find the depths of the colony conducive to growth. Presumably, in every biofilm, there are niches in which certain cells are metabolically quiescent. These cells, having fewer metabolic points of attack, are less susceptible to the action of antimetabolites like antibiotics and disinfectants.

Phenotypic Versatility

Within moments of the time a bacterial cell adheres to a surface it undergoes a remarkable change. Many genes are repressed and others are induces the sum amounting to as much as 40 percent of the total bacterial genome. Many of the newly synthesized proteins play a role in antimicrobic resistance. Some constitute efflux pumps capable of excreting antibiotics at a rate that keeps the concentration below lethal levels.

Metabolic Quiescence

It has long been known that cells that are not doing much are more resistant to antimicrobics than metabolically active ones. In theory there are fewer points of attack in such cells. Nutrient deprivation in stationary phase or in the depths of a biofilm may increase resistance to biofilms by reducing metabolic activity thus depriving antimicrobics of targets.

Persister Cells

Investigator Kim Lewis has found evidence of genes that in a small proportion of the biofilm population produce "toxins" that inhibit certain critical metabolic funcThese genes produce "toxins" that inhibit functions such as Translation and DNA replication.

With these vital functions inhibited, cells expose fewer critical targets to antibiotics and disinfectants and are thus much more resistant. Lewis finds these "persister cells" formed in late log phase planktonic culture as well but in the planktonic state these persisters are destroyed by phagocytic neutrophiles or protozoa. In biofilms the persisters are protected from predation as stated earlier.

Originally thought to be involved in scheduled cell death (Apoptosis-like), these genes are now viewed as producing altruistic persister cells whose primary function is survival. Following exposure to an antimicrobic which kills much of the biofilm, these persisters reactivate by producing antitoxin proteins that inactivate the toxins and permit resumption of metabolic activity and growth. These persister cells are not mutants since cells surviving antibiotic treatment produce once again a population of normally antibiotic cells and a small fraction of recalcitrant persisters.