under the microscope
By Paul J . Pearce , PhD
A Fundamental Consideration for Effective Healthcare Hygiene – Microbial Pathogenicity
Although there have been no studies which definitively link the transmission of an active infectious organism from the environment to a patient , there are numerous references that the environment is a contributor to healthcare-associated infections ( HAIs ).
How does the environment contribute to microbial transmission ? This is not definitely known . However , research findings have shown that a patient admitted to a room previously occupied by a methicillin-resistant Staphylococcus aureus ( MRSA ) – positive or a vancomycin-resistant Enterococci ( VRE ) are at a significantly increased risk of acquiring MRSA and VRE . Similar findings have been reported for patients occupying a room previously occupied by a patient with Clostridioides difficile .
How do microorganisms induce disease after a patient comes in contact with a pathogenic or potentially pathogenic microorganism ?
Pathogenicity is the capacity of a microorganism to initiate disease . It requires the attributes of transmissibility or communicability from one host or reservoir to a fresh host ; survival in the new host ; infectivity or the ability to breach the new host ’ s defenses ; and virulence , a variable that is multi-factorial and defines the ability of a pathogen to harm the host . Virulence in the clinical sense is a manifestation of a complex bacterial – host relationship in which the capacity of the organism to cause disease is considered in relation to the resistance of the host .
Microbial pathogens can be classified into two broad groups , primary and opportunistic pathogens . Primary pathogens can establish infection and causing disease in previously healthy people with intact immunological defenses . However , these bacteria may more readily cause disease in people with impaired defenses . Opportunistic pathogens rarely cause disease in people with intact immunological and anatomical defenses . Only when such defenses are impaired or compromised , because of congenital or acquired disease or using immunosuppressive therapy or surgical techniques , are these bacteria able to cause disease . Many opportunistic pathogens , such as coagulase negative staphylococci and Escherichia coli , are part of the normal human flora and are carried on the skin or mucosal surfaces where they cause no harm and may have beneficial effects , by preventing colonization by other potential pathogens . However , introduction of these organisms into anatomical sites in which they are not normally found , or removal of competing bacteria using broad-spectrum antibiotics , may allow their localized multiplication and subsequent development of disease .
The above classification is applicable to most pathogens ; however , there are exceptions and variations within both categories of bacterial pathogens . Different strains of any individual bacterial species can vary in their genetic makeup and virulence . For example , the majority of Neisseria meningitidis strains are harmless commensal bacteria and considered opportunistic pathogens , however , some hypervirulent clones of the organism can cause disease in a previously healthy individual . Conversely , people vary in their genetic make-up and susceptibility to invading bacteria . For example , Mycobacterium tuberculosis is a primary pathogen but does not cause disease in every host it invades .
Steps in the Pathogenic Process ( Pathogenesis )
The process of pathogenesis involves various steps beginning with the transmission of the infectious agent to the host , followed by colonization of a patient ’ s body ( e . g ., skin , blood , urine ). After the colonization of the patient , the bacteria remain adherent at the site of colonization followed by invasion of the patient ’ s system ( s ). After invasion and surviving the patient ’ s immune system it is ready to cause the disease . Steps involved in the pathogenesis of the bacteria include :
Transmission : Potential pathogens may enter the body by various routes , including the respiratory , gastrointestinal , urinary or genital tracts . Alternatively , they may directly enter tissues through insect bites or by accidental or surgical trauma to the skin . Many opportunistic pathogens are carried as part of the normal human flora , and this acts as a ready source of infection in the compromised host . For many primary pathogens , however , transmission to a new host and establishment of infection are more complex processes .
Colonization : The establishment of a stable population of bacteria on the host ’ s skin or mucous membranes is called colonization . For many pathogenic bacteria , the initial interaction with host tissues occurs at a mucosal surface and colonization normally requires adhesion to the mucosal cell surface . This allows the establishment of a focus of infection that may remain localized or may subsequently spread to other tissues .
Adhesion : Adhesion is necessary to avoid innate host defense mechanisms such as peristalsis in the gut and the flushing action of mucus , saliva and urine , which remove non-adherent bacteria . For bacteria , adhesion is an essential preliminary to colonization and then penetration through tissues . Successful colonization also requires that bacteria acquire essential nutrients for growth . Many bacteria express pili ( or fimbriae ) which are involved in mediating attachment to mammalian cell surfaces .
Invasion : Invasion is penetration of host cells and tissues ( beyond the skin and mucous surfaces ), and is mediated by a complex array of molecules , often described as “ invasins .” These can be in the form of bacterial surface or secreted proteins which target host cell molecules ( receptors ). Once attached to a mucosal surface , some bacteria , e . g ., Corynebacterium diphtheriae or Clostridioides tetani , exert their pathogenic effects without penetrating the tissues of the host . These produce biologically active molecules such as toxins , which mediate tissue damage at local or distant sites .
Paul J . Pearce , PhD , leads The Pearce Foundation for Scientific Endeavor .
References :
Manual of Environmental Microbiology , Fourth Edition . 2016 . ASM . Healthcare Environmental Cleaning . Second Edition . 2012 . AHE . Pathogenesis of Bacterial Infections . 2020 .
https :// nios . ac . in / media / documents / dmlt / Microbiology / Interaction of Various Components of Staphylococcus aureus . Pearce , Paul J . 1973 .