Hello, my name is Oana Ciofu and I am an associate professor of Medical Microbiology at the Faculty of Health and Medical Sciences at University of Copenhagen. In this lecture I will try to answer the question “How are bacteria causing disease?” The mechanism that causes a disease is called pathogenesis. The word comes from the Greek pathos ("disease") and genesis ("creation") The pathogenesis of an infection is the result of complex bacterial-host interactions that can be summarized in 4 main steps: The microorganism (bacteria and fungi) has to: 1. penetrate the normal barriers of the body (skin or mucosa) and adhere to receptors 2. survive the several filters of the defense system of the body: the immune system (such as antibodies, polymorphonuclear neutrophils (granulocytes), mononuclear phagocytes from the lung, spleen and from other organs) 3. multiply and recruit immune cells at the infection site 4. the final step is elimination of the microorganism by the recruited immune cells and antibiotics An acute infection is by definition a short term infection, usually lasting under six months, with sudden onset and usually the microbial pathogen replicates fast. In acute infections the virulence factors play a major role in pathogenesis, causing tissue destruction and recruitment of the immune response. Some examples of acute bacterial infections are: acute uncomplicated urinary tract infection, acute pneumonia, acute diarrhea, meningitis, sepsis. In spite of an immune system that is well functioning, the elimination step is unefficient in some cases and causes chronic or permanent infections. A common cause of an impaired elimination of the microorganism is when microorganisms organize themselves in biofilms. Biofilms are difficult to eliminate due to their tolerance to immune system and antibiotics, as it will be explained latter during this course and cause chronic infections. The course of the infection (acute or chronic) is independent on the type of the microorganisms, as most, if not all, microorganisms can grow as both free-flowing (planktonic cells) or clusters (biofilms) and are able to cause acute and chronic infections, as it will be illustrated later in a case presentation. While the bacterial virulence and/or an impaired immune system play a major role in the establishment of an acute infection, a variety of other host factors can predispose to biofilm-related infections Examples of host factors that predispose to biofilm-related infections are: 1. The presence of foreign-materials like intra- or extravascular devices. It has been shown that the risk of biofilm-infection increases >100.000 times in the presence of foreign-materials in the body 2. Necrotic tissue due to impaired blood supply (vascularization) is also increasing the risk of microbial adherence with subsequent biofilm formation (chronic wounds). 3. Impairment of some components of the innate defense system involved in elimination of the microorganisms, such as: - poor functionality of the cilli on the mucosal membrane of the respiratory tract (cystic fibrosis, primary cilli dyskinesia) - impairment of the function of PMNs present locally at the site of infection (implant-associated chronic osteomyelitis) 4. Lack of effective antibiotic treatment due to an undiagnosed infection or antibiotic treatment in insufficient dosages or for short periods of time being insufficient to eradicate the acute infection. This is mainly the case of infections in difficult to reach sites leading to chronicity of acute infections like the case of chronic otitis media or chronic osteomyelitis. Once established in a biofilm, the bacteria adapt to the host, making them less invasive and less virulent and more persistent (they down-regulates many virulence factors such as toxins, type III secretion systems, motility that are important for the establishment of the acute infection). Biofilms have been found in pacemakers, heart valves, vascular grafts and stents, artificial joints and pins, and breast or other kind of implants Endocarditis Chronic osteomyelitis Chronic otitis media Chronic sinusitis Chronic wounds Chronic lung infection in cystic fibrosis Infectious kidney stone Foreign-body associated infections: intra- and extravascular catheters Bacteria in biofilm are also detaching and therefore a mixture of planktonic, detached cells and biofilm bacteria exist during biofilm infections. The detached, planktonic cells can seed an infection in other location and cause secondary acute or chronic infections. I would like to present a case story of a patient with acute and chronic infection caused by the same microorganism: Staphylococcus aureus A 65 years old overweight, diabetic patient with diabetic wound on the left foot is admitted to the hospital in poor clinical conditions with low blood pressure and fever 39.9 0C. The patient has undergone 5 years ago a knee replacement with a knee implant due to arthrosis of the left knee. The patient has increased inflammatory parameters such as high C reactive protein and leukocytosis and the blood cultures are positive for Gram-positive cocci in clusters that are identified as S. aureus. A staphylococcal sepsis is diagnosed. He is treated with antibiotics for 10 days and he is dismissed from the hospital in good clinical condition. Two months after he has been dismissed from the hospital for the staphylococcal sepsis, he feels pain in his left knee. He is consulted by an orthopedic surgeon who finds a loose prosthesis, inflammatory markers were raised and imaging studies were consistent with bone infection (osteomyelitis). It is decided that the prosthesis has to be removed and a 6 weeks antibiotic treatment started. The infected prosthesis is removed and sent to microbiological laboratory for culturing. The prosthesis is sonicated to disrupt the biofilm and S. aureus is cultured. This patient has been admitted with an acute S. aureus infection: S. aureus in the blood (sepsis) with entrance port the infected diabetic wound (S. aureus grows in biofilms in the chronic wound infection-see later). From the wound, the bacteria enters the lymphatic vessels, pass through the local lymph nodes and enter the blood. The virulence factors allow the bacteria to survive and toxin production lead to inflammatory cytokine and signs of sepsis. Through the blood (hematogenous way) the bacteria reaches the patients knee implant and it attaches to the plastic prosthesis of the knee. After adherence on the implant via fibronectin receptors, S. aureus forms a biofilm (bacteria embedded in slime having a low metabolic rate). The presence of an orthopedic implant also causes a local polymorphonuclear cell defect, with decreased ability to kill phagocytosed bacteria (impaired host factors). This combination of bacterial and host factors lead to impaired elimination of the bacteria and causes a chronic infection. During the course of infection, bacteria induce local bone destruction (osteolysis due to surface proteins of S. aureus) and it is responsible for the septic loosening of the knee implant. For his sepsis infection, the patient is treated with antibiotics administered intravenously. The antibiotics are able to kill the free-flowing bacteria (planktonic cells) in the blood but not the biofilm on the knee implant. The bacteria persist causing a chronic osteomyelitis manifested as pain at the prosthesis site, two months after the antibiotic treatment is stopped. Without removal of the implant, the chronic osteomyelitis infection can be a life-long infection with minimal or no symptoms due to low inflammatory response. Reactivation of osteomyelitis after 50 and 80-years has been reported, underlining the importance of implant removal, the only efficient way to remove the biofilm. In conclusion, this case history illustrates that the bacteria can adopt both living forms: planktonic and biofilm mode of growth. The planktonic growth allows the bacteria to spread and the pathology of the acute infection is mainly related to toxin production and subsequent inflammatory response (sepsis), while the biofilm mode of growth allows the bacteria to persist both in the environment, creating a reservoir for opportunistic pathogens but also in the body, causing chronic infections (in this case chronic wound and osteomyelitis).
