Biofilm, healthcare-associated infections and antimicrobial resistance

Biofilm is an association of micro-organisms in which microbial cells adhere to each other on a living or non-living surfaces, within a self-produced matrix of extracellular polymeric substance (EPS). Biofilm formation is a multi-step process starting with attachment to a surface then formation of micro-colony that leads to the formation of threedimensional structure and finally ending with maturation followed by detachment. During biofilm formation many species of bacteria are able to communicate with one another through specific mechanism called quorum sensing. It is a system of stimulus to co-ordinate different gene expression. Bacterial biofilm is less accessible to antibiotics and human immune system and thus poses a big threat to public health.
Both Gram-positive and Gram-negative bacteria can form biofilms on medical devices, but the most common forms are Enterococcus faecalis, Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus viridans, E. coli, Klebsiella pneumoniae, Proteus mirabilis and Pseudomonas aeruginosa. Also Candida species has the ability to form biofilms.
It is estimated that about 65% of all bacterial infections are associated with bacterial biofilms.These include both, device- and non-device-associated infections.
Biofilms are of great importance because of their ‘resistance’ to antimicrobial therapies.
One of the theories aimed at understanding the “resistance” involves the slow or incomplete penetration of antimicrobial agents through the EPS matrix of the biofilm. The matrix barrier can also act as a defence mechanism against other external stimuli such as UV light and dehydration. The EPS matrix has also been shown to neutralize and dilute antimicrobial substances. Indeed, it has been reported that mature biofilms (over 7 days old) are resistant to 500–5000 times the concentration of bactericidal agents necessary to successfully kill planktonic cells of the same organism. Although incomplete penetration of the matrix barrier has been well recorded and reviewed, this resistance mechanism is not effective against all antimicrobials. Another theory involves the slow growth rate within areas of the biofilm, which is thought to hamper the actions of many antimicrobials that require a certain degree of cellular activity in order to function. It has also been suggested that phenotypic variants commonly referred to as ‘persister cells’ confer resistance within the biofilm owing to their slow rate of growth. Although these persister cells lack the genetic traits that resemble those of antibiotic resistance, they show high levels of multidrug tolerance. Other mechanisms that are thought to play a role in the antimicrobial resistance acquired by certain micro-organisms within biofilms include the presence of efflux pumps, with the expression of several gene-encoding efflux pumps being increased in biofilms. Furthermore, plasmid exchange occurs at a higher rate in biofilms, increasing the chances of developing naturally occurring and antimicrobial-induced resistance. Finally, it is thought that an altered micro-environment within a biofilm, such as nutrient depletion and reduced oxygen levels, may also reduce the efficacy of antimicrobials.

Biofilms play a pivotal role in healthcare-associated infections (HAIs), especially those related to the implant of medical devices, such as ventilator-associated pneumonia, central-venous-catheter-related bloodstream infections, catheter-associated urinary tract infection and surgical site infections.
The initial contamination of the medical device most likely occurs from a small number of micro-organisms, which are often transferred to the device in question via the patient’s or healthcare workers’ skin, contaminated water or other external environmental sources.
Central venous catheters (CVCs) are integral to the modern clinical practices and are inserted in critically-ill patients for the administration of fluids, blood products, medication, nutritional solutions, and for hemodynamic monitoring. They are the main source of bacteremia in hospitalized patients and therefore should be used only if they  are really necessary. About half of nosocomial bloodstream infections occur in intensive care units, and the majority of them are associated with intravascular device. The catheter itself can be involved in 4 different pathogenic pathways:

  • colonization of the catheter by microorganisms from the patient’s skin and occasionally the hands of healthcare workers,
  • intraluminal or hub contamination,
  • secondary seeding from a bloodstream infection, and, rarely,
  • administration of contaminated infusate or additives

Both the outer part of the catheter and the catheter lumen can become contaminated, so resulting in biofilm formation, with the duration that the catheter is in situ impacting on the location and the degree of colonization. Biofilm provides bacterial cells the ability to survive antimicrobial agents and the host immune system and to disseminate to other sites of the body. The best preventive strategy is to avoid any unnecessary catheterization or to reduce indwelling duration when a CVC is required.
The indwelling urethral catheter is an essential tool for many hospitalized patients. It is placed for a number of reasons, including output monitoring of unstable patients, voiding management for patients with urethral obstruction, and perioperative use for selected surgical procedures. However it may carry predictable and unavoidable risk of urinary tract infections perturbing host defense mechanisms and providing easier access of uropathogens to the bladder. For patients undergoing catheterization, the risk of developing a catheter-associated infection increases by approximately 10 % each day the catheter is in place. Catheter-acquired urinary infections may be extraluminal or intraluminal. Extraluminal infection occurs via entry of bacteria into the bladder along the biofilm that forms around the catheter in the urethra. Intraluminal infection occurs due to urinary stasis because of drainage failure, or due to contamination of the urine collection bag with subsequent ascending infection. Extraluminal is more common than intraluminal infection. Biofilms can readily develop on both the inner and outer surfaces of urinary catheters. In order to prevent such infections, it is important for clinicians to utilize catheters only when necessary and to avoid catheterization for extended periods of time.
Ventilator-associated pneumonia (VAP) is a common and serious nosocomial infection in mechanically ventilated patients and results in high mortality, prolonged intensive care unit and hospital-length of stay and increased costs. VAP  has major implications for both the patient and the healthcare system, leading to prolonged hospital stay and increased healthcare costs. VAP has been reported to be prevalent after 48–72 h in patients who have been intubated and are on mechanical ventilation. Biofilm in endotracheal tubes (ET) of ventilated patients has been suggested to play a role in the development of VAP. The role of the ET in VAP pathogenesis became more prominent over the last decades, along with extensive research dedicated to medical device-related infections and biofilms. In order to reduce VAP incidence, it is imperative to better understand the involved mechanisms and to identify the source of infection.
In the setting of the general surgery units a very fearsome complication is the mesh infection. The use of a mesh has become the standard in hernia repair surgery worldwide due to the reduced rates of recurrence and technical ease of the operation. However, mesh-related complications have become increasingly more frequent. Post-operative mesh infections are rare but troublesome complications that cause considerable morbidity and necessitate mostly mesh removal. Antibiotics and mesh-saving operations are not generally sufficient to eradicate the infection in the majority of cases.
Bacterial adherence and biofilm formation on the surface of synthetic materials are essential steps in the sequence leading to mesh infections. Once biofilm is formed, complete removal of the implant is nearly mandatory for the eradication of the infection. Several studies have documented in vitro that multiple species of bacteria can attach to prosthetic mesh surfaces and form biofilms. The nature of biofilm structure makes micro-organisms difficult to eradicate and confer an inherent resistance to antimicrobial agents. Although, several studies have shown that in certain instances a conservative approach may be successful for salvaging a contaminated mesh, in most cases antibiotics and wound drainage are not sufficient to eradicate the infection. If conservative treatment fails, the complete surgical removal of the mesh is mandatory.
To date, there are no detection methods available for the diagnosis of a biofilm within a clinical setting. The use of traditional culture methods to determine colonization is not indicative of biofilm growth. Furthermore, negative results from swab samples may not necessarily imply the absence of an infection, but could possibly be due to the slow growth rate within a biofilm of species that cannot be detected within the usual detection range.
In conclusion, biofilms are of great importance in the control of healthcare-associated infections. This is not only due to their ability to act as a safe-haven for those microorganisms that are of public health significance, but also due to their inherent ‘resistance’ to antimicrobials. Moreover, diagnosing a biofilm infection represents an area of grave concern.
As a consequence, the best way to prevent biofilm formation is to prevent healthcare-associated infections!