Surgical microbiology and antimicrobial therapy

Chapter 14


Surgical microbiology and antimicrobial therapy



Chapter objectives


After studying this chapter, the learner will be able to:


• Differentiate between microorganisms.


• Describe the natural lines of defense of the human body.


• Identify several favorable living characteristics of microorganisms.


• Describe how disease is spread.



Key terms and definitions



Aerobic 


Microorganism that requires air or the presence of oxygen for maintenance of life.


Anaerobic 


Microorganism that grows best in an oxygen-free environment or one that cannot tolerate oxygen (e.g., Clostridium species that causes gas gangrene).


Antibiotics 


Substances, natural or synthetic, that inhibit growth of or destroy microorganisms. Used as therapeutic agents against infectious diseases; some are selective for a specific organism; some are broad-spectrum antibiotics.


Antimicrobial agent 


Chemical or pharmaceutical agent that destroys or inhibits growth of microorganisms.


Biofilm 


Three-dimensional layers of living bacteria embedded in a sticky matrix that persists on the surface of tissues and implanted medical devices. Commonly the cause of chronic infections, such as otitis media and rhinitis.


Bioterrorism 


Covert event involving introduction of microbial contamination and infection of humans or animals.


Community-acquired infection 


Infectious disease process that developed or was incubating before the patient entered the health care facility.


Cross-contamination 


Transmission of microorganisms from patient to patient and from inanimate objects to patients and vice versa.


Endospore 


Forms of bacterial classes clostridia and bacillus that are generated when living conditions are not favorable. Protective capsule that forms inside specific bacterial species encircles and protects the genetic matter to resist destructive forces, such as disinfection or sterilization.


Epidemiology 


Study of occurrence and distribution of disease; the sum of all factors controlling the presence or absence of a disease.


Florae 


Bacteria and fungi normally inhabiting the body, resident or transient.


Hospital-associated or acquired infection (HAI) 


Not present when the patient was admitted to the health care facility. Infection may occur at the surgical site or as a complication unrelated to the surgical site (formerly known as nosocomial infection).


Infection 


Invasion of the body by pathogenic microorganisms and the reaction of tissues to their presence and to toxins generated by the organisms.


Microorganisms 


Living organisms, invisible to the naked eye, including bacteria, fungi, viruses, protozoa, yeasts, and molds.


Opportunists 


Microorganisms that do not normally invade tissue but are capable of causing infection or disease if introduced into the body mechanically through injury, such as tetanus bacillus, or when resistance of the host may be lowered, as by human immunodeficiency virus (HIV) infection. Opportunistic infection.


Pathogenic 


Producing or capable of producing disease.


Prion 


Protein that contains no genetic material and in its pathologic form causes fatal neurodegenerative disease. Difficult to inactivate. Requires special handling.


Sepsis 


Severe toxic febrile state resulting from infection with pyogenic microorganisms, with or without associated septicemia. Septicemia is a clinical syndrome characterized by significant invasion into the bloodstream of microorganisms from a focus of infection in tissues. Microorganisms may multiply in the blood. Infection of bacterial origin carried through the bloodstream is sometimes referred to as bacteremia.


Standard precautions 


Procedures followed to protect personnel from contact with blood and body fluids of all patients (formerly referred to as universal precautions).


Superinfection 


Secondary subsequent infection caused by a different microorganism that develops during or after antibiotic therapy.



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Microorganisms: nonpathogens versus pathogens


Specific numbers of microorganisms with plant like or animal-like characteristics are considered nonpathogenic in the human if they are not transferred to a different location in the body. Nonpathogenic microorganisms do not pose a particular threat to health if they remain constant in microbial numbers. Natural resident florae can be found in the reproductive tract secretions, gastrointestinal tract, nasopharyngeal mucus, respiratory passages, and any superficial ductal opening, such as sweat and oil glands.


Resident florae have specific roles in their natural location. Some resident florae of the large intestine aid in the synthesis of vitamin K, vitamin B12, and folic acid, but can cause an infection if they are relocated to a surgical incision.


The term infection is used when nonresident florae invade a susceptible area. The term superinfection is used when resident florae are out of balance and the increased number causes a pathogenic condition. If the count of microbial colonies increases above normal or the colonies grow in an area where they are not usually found, an infection results and the microorganisms are considered pathogenic. They can invade healthy tissue through some power of their own or can injure tissue by producing a toxin. Pathogenic microorganisms can cause sepsis—a severe toxic febrile state—or death.


In a healthy state, body fluids such as urine and cerebrospinal fluid do not normally contain microorganisms and are considered sterile. Other fluids that under normal circumstances do not contain microorganisms include blood, peritoneal fluid, synovial fluid, amniotic fluid, tears, semen, and breast milk. Because microorganisms are not visible on gross inspection, exposure to any body substance, including respiratory exhalations, should be considered contaminated and treated accordingly.



Identification of microorganisms


Microorganisms, with the exception of viruses, have intracellular DNA. The DNA is either enclosed in a nuclear membrane (eukaryotic) or loose in the cytoplasm (prokaryotic). Bacteria and blue-green algae are the only prokaryotic microorganisms. Bacteria remain in a primitive state unchanged by evolution. All other microorganisms are eukaryotic and have changed many times on an evolutionary scale.


Specimens and cultures of tissues or body fluids may be sent to the microbiology laboratory for identification. Treatment is prescribed according to the type of microorganism present in the body. The type of substance taken from the body for testing may be a clue as to the type of microorganism.


Accurate identification of the microorganism is critical to the selection of the appropriate therapy. Criteria used by laboratory personnel to identify the type of microorganism include the following:




Viability of microorganisms


Any microorganism can become a pathogen when transferred from one place to another. Each type of microorganism has its own method of duplicating itself. Epidemiology is the study of how microorganisms multiply and spread.


Bacteria pass through four phases during their colonization/duplication/reproductive stage:



The rate of bacterial growth is depicted in Figure 14-1. The population of microbes will double during the log phase at a predictable interval referred to as generation time or doubling time. Changes in temperature, moisture, illumination, nutrients, and pH can influence the rate at which any microbe passes through doubling time. If all favorable living conditions are met, the microbe can proliferate into disease state. Bacteria secretes a slime that accumulates in layers known as biofilm.


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FIG. 14-1 Logarithmic growth of a bacterium.

Any change in the favorable conditions can alter the amount of time it takes for the microbe to pass through any one of the four phases. Box 14-1 describes the favorable living conditions associated with the proliferation of microorganisms.



BOX 14-1   Favorable Living Conditions for Microorganisms




The essential element in preventing pathogenic disease is to understand the microorganism’s mode of transmission, life cycle, and favorable living conditions. Interference with the viability, doubling, and survival of pathogenic microorganisms can minimize the risk of infection.



Three lines of defense


The human body is remarkable in its ability to protect itself by intact barriers, membranes, and bacteriostatic secretions. Three lines of defense are particularly important for prevention of disease.1,4


The first line of defense involves generalized good health and incorporates natural biochemical, mechanical, and anatomic protection through the following:



• Skin. Stratified epithelium contains sweat (sudoriferous) and oil (sebaceous) glands, which are bactericidal. Natural florae inhibit each other. The epithelium must remain intact to afford protection. Desquamation and low pH impede bacterial colonization.


• Mucous membranes. An effective barrier when intact, these line all natural body orifices except the ears, which secrete cerumen (ear wax). Mucous membranes have bactericidal properties and a slightly acidic pH.


• Reflexes. Examples include vomiting, gagging, and blinking.


• Sneeze. Mucociliary escalator of the respiratory tree moves mucus and debris from the respiratory tract.


• Genitourinary/reproductive tracts. Immunoglobulin A (IgA), found in mucus, and enzymes are produced; these have a slightly acidic pH. Peristalsis is unidirectional.


• Eyes. Enzymes and IgA protect the conjunctiva and structures of the eye.


• Cellular level. Interferon is naturally formed in the cell to fight against viral attack.


• Stomach acid. Acid kills most pathogens.


• Muscular closure of orifices. Muscular closure provides a mechanical barrier against orifices such as the cervix and sphincters.


The second line of defense involves the collaborative effort of several body systems to prevent the proliferation of pathogenic microorganisms. These provide secondary protection if the microorganism breaks through the first line of defense. The second line of defense includes the following:



The third line of defense can be acquired naturally or induced therapeutically. The third line of defense requires actual exposure to the pathogen in some form during which temporary or quasi-permanent resistance is attained. Methods include the following:




Pathogenic invasion


Pathogenic microorganisms initially invade and aggregate in a body system or at a localized site, such as an abscess. The proliferation of infectious material is easily supported by the warmth, chemical composition, moisture, and other components of the body.


Local spread is supported by the surface of the wound, necrosis of tissue, diminished or ineffective inflammatory response, and absence of anatomic barriers. Other pathways of infection include any body orifice, duct, or lumen of a broken blood vessel.


Veins are particularly vulnerable, because they are often associated with venous sinuses and have low pressure. Venous stasis permits the growth of microbial colonies. The central nervous system (CNS) shares a similar risk for infection because the cerebrospinal fluid is under lowered pressure and has components that are rich in nutrients. As microbial colonies increase in number, infection may be carried through the body in the lymphatic system and may eventually become bloodborne. Major organ systems can become involved and can result in multisystem organ failure and death. Any body fluid or substance is a potential carrier of pathogens (Box 14-2).



BOX 14-2   Body Fluids and Substances That May Transmit Pathogens




According to the CDC, 14% to 16% of all health care–associated infections are surgical-site infections (SSI). Approximately 77% of surgical patients who die are reported to die of sepsis associated with these infections. SSI increases the length of stay and increases the cost of care.5


Knowledge of how the cycle of infection works is the most important element of prevention. Considerations include but are not limited to the following:




Infectious processes in the body


Clinical infection is the product of the introduction, metabolic activities, and pathophysiologic effects of microorganisms in living tissue. It can develop in the surgical patient as a preoperative complication after an injury as a community-acquired infection (CAI) or as a postoperative complication acquired in the hospital referred to as a health care–acquired infection (HAI).


A localized HAI infection may begin at the surgical site between the fourth and eighth postoperative days. Infection, usually bacterial in origin, develops as a diffuse, inflammatory process, known as cellulitis, and is characterized by pain, redness, and swelling. This inflammatory response is the body’s second line of defense directed toward localization and containment of the infecting organism after it has passed through the first line of defense.


Red blood cells, leukocytes, and macrophages infiltrate the affected area, with pus formation (suppuration) often following. An abscess forms as a result of tissue liquefaction with pus formation, supported by bacterial proteolytic enzymes that break down protein and aid in the spread of infection. Fibrinolysin, for example, an enzyme produced by hemolytic streptococcus, may dissolve fibrin and delay surgical-site healing.


The body attempts to wall off an abscess by means of a membrane that produces surrounding induration (hardened tissue) and heat. Localized pus should be drained promptly.


If localization is inadequate and does not contain the infectious process, spreading and extension occur, causing regional infection. Microorganisms and their metabolic products are carried from the primary invasion site into the lymphatic system, spreading along anatomic planes and causing lymphangitis. Failure of the lymph nodes to hold the infection results in uncontrolled cellulitis. Subsequently, regional and/or systemic infection may develop, characterized by chills, fever, and signs of toxicity.


Septic emboli may enter the circulatory system from septic thrombophlebitis of regional veins communicating with local infections. These emboli and pathogenic microorganisms in the blood seed invasive infection and abscess formation in remote tissues.


Sepsis elevates the patient’s metabolic rate 30% to 40% above average, imposing additional stress on the vital systems. For example, cardiac output is about 60% above normal resting value. The body’s defenses and ability to meet the stress govern whether the infectious process progresses to systemic infection or septic shock. Multiple infection sites, the presence of shock, and inappropriate antibiotic therapy result in a poor prognosis.


Obviously the key to avoiding infection is prevention, but if an infection begins, the next most important step is to identify the microorganism and treat it appropriately. More than one microorganism may be present; therefore treatment is highly individualized. Diagnosis of systemic infection may include any two of the following:



Septic shock may include decreased urinary output (less than 30 mL/hr), altered mental state, and hypoxemia. The patient usually becomes very hypotensive (low blood pressure) and has signs of blood-clotting defects, such as bruising.


The ultimate resolution of infection depends on immunologic and inflammatory responses capable of overcoming the infectious process. This is associated with drainage and removal of foreign material, including debris of bacteria and cells, lysis (breakdown) of microorganisms, resorption of pus, and sloughing of necrotic tissue. Healing then ensues.



Who is at risk for exposure?


Perioperative personnel who provide direct and indirect patient care are at risk for exposure to potentially harmful microorganisms. Wearing personal protective equipment (PPE), such as gowns, gloves, and eyewear with side shields, decreases the risk but does not eliminate it. The risk for exposure is proportionate to the proximity to the patient in the OR.


The closer to the surgical field (source of blood and/or body substance) one is, the higher the risk of exposure. The surgeon, assistants, and scrub persons have a higher risk by role and proximity. They share an increased incidence of needlesticks and puncture wounds. The circulating nurse, environmental services personnel, and instrument processors are also at increased risk for body substance exposure because of specimen handling, cleaning processes, and other contaminants in the environment.


Exposure rates to blood and body substances for OR personnel have been reported as 10 per 100 procedures. Sharps were responsible for 3 of 100 exposures reported. Of glove tears reported, 93% were in single-gloved caregivers. Approximately 63% of glove tears in a single-gloved individual revealed a blood exposure. In 20% of double-gloved individuals who had a glove puncture, only 6% had evidence of inner-glove puncture. Double-gloving is not an assurance of avoiding puncture in the event of a needlestick. In 74% of injuries with sharps, the injuries were self-inflicted by carelessness.


The patient is also at risk. If a needlestick occurs, the needle may come into contact with the patient after penetrating the caregiver, thereby exposing the patient. Some patients have health conditions that predispose them to vulnerability for infection. Considerations related to higher risk include immunosuppression, an immature immune system (preterm and term infants), radiation therapy, burns, diabetes, nutritional depletion, smoking, chemotherapy for cancer, older patients, steroid use, sickle cell disease, alcoholism, liver and kidney disease, and preexisting infection being treated with antibiotic therapy (superinfection/opportunistic infection may ensue).



Biofilm


Biofilm forms when one or more species of bacteria, fungi, and other microorganisms adhere in layers to moistened surfaces, such as biologic tissue, implantable metals, and plastics (Box 14-3). The slimy matrix that binds the microorganisms together creates a barrier against antibiotic treatment that results in a persistent disease state. Plaques can break off and attach elsewhere, causing a separate biofilm colony.



BOX 14-3   Examples of Microbiologic Biofilms Associated with Chronic and Resistant Infections


Bacteria





Biofilm can form on any biologic tissue surface or on inert devices implanted in the body, such as catheters, artificial joints, and mechanical heart valves. The biofilm begins as a single cell layer that attaches to a surface and multiplies and thickens rapidly (Fig. 14-2). Research has shown that biofilm is a genetically mediated process in which bacteria exchange intercellular information that give rise to newly formed biofilm in a process known as quorum sensing.


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FIG. 14-2 Biofilm life cycle. The process can be broken down into conditioning film formation and initial attachment of the organism to the surface, growth, and development of the biofilm, and organism detachment and dispersal. (From P. Dirckx, Montana State University Center for Biofilm Engineering. Reproduced with permission.)

Persistent bacterial cells contain a gene (HipA) that codes for a toxic protein, which puts the cell into hibernation until the effects of a specific antibiotic have worn off. Antibiotics work only on growing, animated cells. When the antibiotic ceases to work, the cells reanimate and repopulate the site. Deleting or deactivating the HipA gene could end the ongoing battle with biofilm.


A biofilm infection may linger for months, years, or even a lifetime, regardless of the state of the patient’s intact immune system. Bacteria in biofilm can be difficult to eliminate. The National Institutes of Health (NIH) points out that 75% of human infections are attributed to biofilm.7 An infected implant may need to be explanted.



Mandatory reporting of health care–acquired infections


The Healthcare Infection Control Practices Advisory Committee (HICPAC) of the Centers for Disease Control and Prevention (CDC) published a document in 2005 on reporting health care–acquired infections (HAIs). Infections to be reported include a wide range of patient-related infections that are traceable to health care intervention. Examples include but are not limited to the following:



Methodology and surveillance activities are described by HICPAC in the recommendations to the CDC. The pros and cons include public reporting of infection rates, but a potential for misunderstanding by the general lay population and misrepresentation of published data.



Types of pathogenic microorganisms


Infections may be caused by one or a combination of microorganisms. Each type of microorganism has a specific set of characteristics that promotes survival and proliferation. Knowing the specific needs for microbial life aids in the prevention of infection. In this chapter, each of the five main types of microorganisms is described according to structure, life cycle, and mode of transmission. Examples of each type of microorganism are provided in Table 14-1.




Bacteria


Bacteria are unicellular microbes essential to human life. We depend on many of their metabolic processes. Many antibiotics are derived from bacteria, such as erythromycin, chloramphenicol, and kanamycin. Some photosynthesizing types convert carbon dioxide to water and oxygen.


More than 5000 species of bacteria have been named, and many more exist unidentified. Most of the morphologic differences in bacteria are found in metabolism, chemical composition, or resultant effect on the host. Unfortunately, many varieties of bacteria are pathogenic or are capable of becoming pathogenic (Fig. 14-3).


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FIG. 14-3 Streptococcus infection.

Bacteria can survive in diverse environments. For example, Thermoplasma acidophilum is found in the hot springs of Yellowstone Park and is capable of living in a 140° F (60° C) environment with an acidic pH of 1 to 2. Geobacillus stearothermophilus endospores (formerly known as Bacillus stearothermophilus) are used to test steam sterilizers because they can withstand temperatures up to 140° F (60° C). Bacillus atrophaeus endospores are used to test dry heat and low-temperature hydrogen peroxide sterilizers because they are destroyed at a lower temperature of 98.6° F (37° C).



Characteristics




1. Structure. Bacteria are microscopic, single-cell structures (1 to 10 mm). Two billion bacteria can be contained in a single drop of water. Bacteria have DNA but no formal nucleus (prokaryotic) and no membrane-bound organelles.



a. Cocci are round. Examples include strains of Staphylococcus and Streptococcus. Diseases include impetigo and gonorrhea.


b. Bacilli are rod shaped, and some can form endospores. They are the most common types of bacteria. Examples include Bacillus, Clostridium, Escherichia, Proteus, and Pseudomonas species.


c. Spirochetes are spiral shaped. Diseases include syphilis, leptospirosis, and Lyme disease. Syphilis and Lyme disease can have fatal neurologic effects in the later stages of illness known as neurosyphilis and neuroborreliosis (Lyme).3


d. Pleomorphs can change shape from rod to round, making positive identification difficult. Diseases include mycoplasmal infection, typhus, rickettsial infection, chlamydia, psittacosis, and Rocky Mountain spotted fever. Rickettsia and chlamydia organisms must live in a host cell. They are intracellular parasites.


2. Life cycle. Bacteria can be aerobic, anaerobic, facultative, or microaerophilic. They can reproduce asexually by binary fission (split into equal halves). Some studies have shown that some genes may be transferred between bacterial species during viral infection (plasmid transfer).


3. Communication. Some bacteria are capable of communicating that conditions are adequate for reproduction and colonization. The process of cells gathering and communicating is known as quorum sensing.7 When the cells have populated an area to a sufficient degree, the process of formal infection takes place. Quorum sensing is commonly found in biofilm accumulations.



4. Transmission. Infection is passed along by direct contact or through animal or insect bites.


5. Encapsulation. Some bacteria are enclosed within a thick wall, which is a defense mechanism against phagocytic activity of leukocytes. These bacteria may be ingested by white blood cells, but instead of being killed and digested, they remain within the phagocyte for a time and are then extruded in a viable condition. The presence of a capsule is associated with virulence among pathogenic bacteria. This is not to be confused with endospore formation, which is discussed in later paragraphs.



Differentiation of bacterial types



Gram stain.

A universal way to determine the differences in bacteria is to stain the cell. Danish physician Christian Gram (1853-1938) developed a method of applying a solution of crystal violet and iodine (gentian violet) to the cell wall (bacterial coat) followed by exposure to 95% alcohol and acetone. Gram-positive bacteria retain a stain of dark purple-blue. Gram-negative bacteria retain only a stain of light pink after the rinsing process (Table 14-2). Gram stain is also used to identify nonbacterial substances such as trophocysts (helminth eggs) and larvae.


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Apr 6, 2017 | Posted by in GENERAL SURGERY | Comments Off on Surgical microbiology and antimicrobial therapy

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