Bloodstream Infections.pdf

Bloodstream Infections Chapter 67 Bailey & Scott’s Diagnostic Microbiology Patricia M. Tille Fourteenth Edition Copyright © 2017 Elsevier Inc. All Rights Reserved Objectives  Define the suffix “emia” -Define septicemia and outline its clinical manifestations  Explain the major types of bloods...

Bloodstream Infections Chapter 67 Bailey & Scott’s Diagnostic Microbiology Patricia M. Tille Fourteenth Edition Copyright © 2017 Elsevier Inc. All Rights Reserved Objectives  Define the suffix “emia” -Define septicemia and outline its clinical manifestations  Explain the major types of bloodstream infections and outline the causes of these infections  Provide a detailed explanation of the process(es) of specimen collection, handling, processing, and culturing for bacteremia detection -Outline the use of anticoagulants in blood cultures  Outline the general guidelines used to distinguish probable pathogens from contaminants in a blood culture Terminologies  The suffix emia is derived from the Greek word meaning “blood” & refers to the presence of a substance in the blood: (Bacteremia, viremia, parasitemia, fungemia) -Bacteremia: bacteria in the blood *Primary bacteremia: Bacteria directly introduced into the blood stream *Secondary bacteremia: Bacteria entered the blood from an infected body site *Occult bacteremia: Bacteria in the bloodstream of febrile children (with no obvious source of infection and illness) *Pseudobacteremia: Due to false blood culture results -Septicemia/toxemia: bacteria/bacterial products in the blood, producing an infection & reproducing in the blood stream *Fever, hypothermia, chills, malaise, hyperventilation, respiratory alkalosis, skin lesions, change in mental status, diarrhea, Rapid heart rate, nausea and vomiting *Systemic inflammatory response *Massive immune response, inadequate blood flow, disseminated intravascular coagulation (DIC), sepsis, septic shock, organ failure, and death Causative Agents of Bloodstream Infection  Bacteria: -Most commonly isolated are Staphylococcus aureus, Streptococcus pneumoniae, and E.coli *Inhabitant of the healthcare environment *Organisms that colonize the skin, oropharynx, and gastrointestinal tract  Fungi: -Primarily in immunosuppressed patients and those with serious or terminal illness Staphylococcus epidermidis -Candida albicans (most common) *Long-term hospitalization, intravascular catheters, diabetes, malignancies -Malassezia furfur *Particularly in neonates receiving lipid-supplemented parenteral nutrition  Parasites: May be found transiently in the -Fungi do not invade blood cells, but their presence in blood indicates a focus on bloodstream as they migrate to other tissues or infection elsewhere in the body organs *Can disseminate to all body organs -Ex: Toxoplasma gondii found in circulating blood -Fungi enter to the blood via: invade lymph nodes and other organs * Loss of integrity of the gastrointestinal or other mucosa (Toxoplasmosis) * Through damaged skin -Plasmodium parasites invade erythrocytes and * From primary sites of infection (ex: pneumonia initiated in lungs) hepatic parenchymal cells (Malaria) *By means of intravascular catheters -Babesia: infects red blood cells (Babesiosis) -Trypanosoma: (Trypanosomiasis)  Viruses: -Primary pathology relates to infection of the target organ or cells -Viruses that infect blood cells are Epstein-Barr virus (lymphocytes), cytomegalovirus (monocytes, polymorphonuclear cells, and lymphocytes), human immunodeficiency virus (HIV) (CD4+ T lymphocytes and macrophages) and other human retroviruses (lymphocytes) Types of Bacteremia  Transient bacteremia: -Asymptomatic and a result of a procedure associated with a non-sterile anatomical site -Normally bacteria are cleared from blood and causes no infection -Septicemia can still occur -Teething infants and people having dental procedures (oral flora enter to the bloodstream through breaks in the gums) -Manipulation of infected tissues, instrumentation (inserted through contaminated mucosal surfaces), surgery (involving nonsterile sites)  Intermittent bacteremia: -Recurrent/bacteria periodically present in bloodstream at various time intervals followed by clearing of that bacteria from the bloodstream -From existing infection elsewhere in the body (Ex: undrained abscesses)  Continuous bacteremia: - when bacteria is released into the blood at a fairly constant rate -In septic shock, bacterial endocarditis, and other endovascular infections -During the early stages of specific infections, including typhoid fever, brucellosis, and leptospirosis Types of Bloodstream Infections  Intravascular: -Originate within the cardiovascular system (Infected heart valves, catheters, shunts, veins) -Continuous bacteremia -Intravascular infections include: *Endocarditis *Mycotic aneurysm *Suppurative thrombophlebitis *Intravenous (IV) catheter-associated bacteremia -Normal oral flora * Infective Endocarditis: -Infection of the endocardium -Most commonly caused by bacteria *Viridans streptococci: Primary causative agents (normal oral flora) -Enter bloodstream as a result of gingivitis, periodontitis, or dental manipulation -Use of IV catheters, arterial lines, and vascular prostheses -infection associated with intravascular catheters -HACEK group *Mycotic Aneurysm and Suppurative Thrombophlebitis: -Result from damage to the endothelial cells lining blood vessels Mycotic aneurysm: *Infection Inflammatory damage and weakening of an arterial wall eventually rupture) *Similar etiologic agents as those that cause endocarditis bulging of the arterial wall (aneurysm) (can Suppurative thrombophlebitis: *Inflammation of a vein wall alteration in the vein’s endothelial lining followed by clot formation *Common complication of hospitalized patients caused by the increasing use of IV catheters *Intravenous (IV) Catheter-Associated Bacteremia: -Central venous catheters are used to administer fluids, blood products, medications, antibiotics, and nutrition, and for hemodynamic monitoring -Catheter is colonized by either bacteria or fungi (major source of health-care associated illness and death) -Most common etiologic agents are organisms found on the skin -Uncommon routes include contaminated fluids or blood borne seeding from another infection site  Extravascular: -Result from bacteria entering the blood circulation through the lymphatic system from another site of infection -Result in most cases of clinically significant bacteremia *Most common routes of entry of bacteremia: Genitourinary tract (25%), respiratory tract (20%), abscesses (10%), surgical wound infections (5%), biliary tract (5%), miscellaneous sites (10%), uncertain sites (25%) *Probability of bacteremia occurring from an extravascular site depends on: site of infection, organism’s pathogenic potential, host defense -Ex: Organism producing meningitis is likely to produce bacteremia at the same time N.B: Organisms other than bacteria (ex: fungi) may also cause intravascular or extravascular infections Factors contributing to bloodstream infections     Immunosuppressive agents Widespread use of broad-spectrum antibiotics Invasive procedures/Extensive surgical procedures Prolonged survival of debilitated and seriously ill patients Detection of Bacteremia  Mortality rates associated with bloodstream infections: 30 to 50% -Timely detection and identification  To detect bloodstream infections: -Obtain blood via aseptic venipuncture and incubate in culture media (Blood culture) - Detect bacterial growth using manual or automated techniques - Once growth is detected: the organism is isolated, identified, and tested for susceptibility to various antimicrobial agents  Preparation of the specimen collection site: Specimen Collection -Antisepsis: Once a vein is selected, the skin site is defatted (fat removal) with 70% isopropyl alcohol and an antiseptic (ex. iodine tincture or chlorhexidine) is applied to kill surface and subsurface bacteria according to the manufacturer’s recommendations Not recommended to draw blood through a vascular shunt or catheter, because these prosthetic devices are difficult to decontaminate completely -Site Care: -Iodine can cause skin irritation; Should be removed with alcohol  Collection from separate sites/using separate needles and syringes -Collection Method: *Needle and Syringe : Transfer into blood culture bottles using same needle *Butterfly needle  Preparation of the specimen container: -Disinfect the top of the culture bottles and tubes  Specimen Volume and number of blood cultures: -Bacteremia in adults, children, and infants can have a low number of colony forming units (CFU) per milliliter (mL) of blood -Direct relationship between the volume of blood and an increased probability of isolating the infecting organism *In adults: Collect 10 to 20 mL of blood per culture *In infants and children: It is not safe to obtain more than 4% to 4.5% of a patient’s known total blood volume for culture (1-5ml) -Relation between blood volume and patient weight -If volume of blood is adequate, usually 2 to 3 blood cultures (sets) are sufficient to obtain optimum sensitivity *Differs if pre or post antimicrobial therapy: -In patients with endocarditis *Who have not received antibiotics: -Single blood culture is positive in 90% to 95% of the cases -Second blood culture establishes the diagnosis in at least 98% of patients *Who have received prior antibiotic therapy: -Three separate blood collections (16 to 20 mL each) -Might need additional blood culture or two on the second day (if necessary) -In patients without infective endocarditis -65.1% detection in first culture, 80% by first two cultures, 95.7% in first three blood cultures Preparation of the specimen container Preparation of the Site Collecting Method Laboratories that recover contaminants at rates greater than 3% should suspect improper phlebotomy techniques phlebotomists should wear gloves for blood drawing Contaminated surfaces that might come in contact with the disinfected venipuncture site should be disinfected  Timing of Collection: The timing of cultures is not as important as other factors in patients with intravascular infections because organisms are released into the bloodstream at a fairly constant rate -Generally accepted to space two or three blood cultures an hour apart -A study found no significant difference in the yield between multiple blood cultures obtained simultaneously or those obtained at intervals  Transport: -Immediate -High priority transportation  Miscellaneous Matters : Anticoagulation: -Blood drawn for culture must NOT be allowed to clot (difficult to recover organisms in clotted specimens) -Blood drawn is inoculated either directly into the blood culture broth media or into a sterile blood collection tube (discouraged) containing an anticoagulant -Invert to mix blood with broth media and anticoagulant -Examples of anticoagulants: *Heparin, ethylenediaminetetraacetic acid (EDTA), citrate: inhibit numerous organisms (not recommended) *Sodium polyanethol sulfonate (SPS) in concentrations of 0.025% to 0.03% (recommended) -0.025% to 0.05% SPS is most commonly used SPS: -Prevents clotting -Inhibits complement system -Anti-phagocytic -Interferes with the activity of some antimicrobial agents, notably aminoglycosides - May inhibit the growth of some strains of Neisseria spp., Gardnerella vaginalis, Streptobacillus moniliformis, and all strains of Peptostreptococcus anaerobius *Addition of 1.2% gelatin counteracts this inhibitory action of SPS, but the recovery of other organisms decreases Dilution blood factor: -Dilute out or inactivate antibacterial components of the blood -Traditionally: 1:10 ratio of blood to medium -New commercial media containing resins or other additives:1:5 ratio 1:5 ratio of blood to unmodified medium is adequate in conventional blood cultures Blood Culture Media: -Contain nutrient broth and an anticoagulant -Most blood culture bottles available commercially contain trypticase soy broth, brain-heart infusion broth, supplemented with peptone, or thioglycolate broth -More specialized broth bases include Columbia or Brucella broth *Resin or charcoal containing medium: -Absorbs and inactivates most antimicrobial agents -Increases the yield of microorganisms over standard blood culture media  Incubation Conditions: -Temperature -Time: 5-7 days on average; longer incubation is required for some microorganisms -Agitation Culture Techniques  Conventional blood cultures *In commercially prepared blood culture bottles: low oxidation-reduction potential (growth of most facultative and some anaerobic organisms) *Transient venting of the bottles with a sterile, cotton-plugged needle may be necessary (growth of obligate aerobes, such as yeast and Pseudomonas aeruginosa) *Constant agitation of the bottles during the first 24 hours of incubation (enhances the growth of most aerobic bacteria) *Bottles examined daily for growth (turbidity, gas, discoloration) *Blind subculture  Biphasic Method/ Self-Contained Subculture System -Liquid and solid medium in the same bottle -To subculture, entire broth content are allowed to contact the agar surface by inverting the bottle -Examined daily for growth -BD Septi-Chek system (Modification of the biphasic blood culture medium) -Consists of a conventional blood culture broth bottle with an attached chamber containing a slide coated agar (Special media for isolation of fungi and mycobacteria are also available) -Faster detection for many organisms compared to conventional systems -Enhance the recovery of Streptococcus pneumoniae -Do not efficiently recover anaerobic isolates  Lysis Centrifugation -To detect microorganisms (bacteria, fungi, mycobacteria) in a specimen of body fluid -The Isolator consists of a stoppered tube containing saponin (lyse blood cells) and SPS (anticoagulant) -Centrifuge -Discard supernatant -Vortex sediment containing the pathogen -Plate entire sediment on solid agar -Benefits of this system: *Rapid and improved recovery of filamentous fungi *Presence of actual colonies for direct identification and susceptibility testing after incubation *Ability to quantify the colony-forming units present in the blood *Rapid detection of polymicrobial infections *Ability to choose special media for initial culture setup based on clinical impression *Potential enhanced recovery of intracellular microorganisms caused by lysis of host cells -Limitations of the system: *Relatively high rate of plate contamination *Decreased ability to detect certain bacteria (Streptococcus pneumoniae, Listeria monocytogenes, Haemophilus influenzae, anaerobic bacteria)  Instrument-Based Systems BACTEC Systems (Fluorescent detection of CO2) -Measures the production of carbon dioxide (CO2) by metabolizing microorganisms -Blood or sterile fluid is inoculated into bottles containing appropriate substances -Fully automated with the incubator, shaker, and detector all in one instrument -Uses fluorescence to measure CO2 released by microorganism -Gas-permeable fluorescent sensor on the bottom of each vial -As CO2 diffuses into the sensor and dissolves in water present in the sensor matrix hydrogen (H+) ions are generated, pH decreases, therefore increases the fluorescent output of the sensor BacT/ALERT Microbial Detection System (Colorimetric detection of CO2) -Measures CO2-derived pH changes with a colorimetric sensor in the bottom of each bottle -Sensor is separated from the broth medium by a membrane permeable to CO2 -As microorganisms grow, they release CO2 which diffuses across the membrane and is dissolved in water present in the matrix of the sensor free hydrogen ions are generated cause color change detected by sensor (blue to light green to yellow) Versa TREK System: -Unique agitation system during blood culture inoculation -Microorganism detection is based on pressure changes in bottle headspace using a pressure sensor -The aerobic media bottles each contain a small magnetic stir bar enhancing oxygenation during incubation -A continuously monitoring instrument (like other automated systems) Handling Positive Blood Cultures -Positive blood cultures can be detected by: Appearance of broth (Turbid, hemolysis, gas production); Colonies on subculture plates; Instrument based detection *Broth-based automated blood culture methods are mostly used -When a positive culture is indicated, perform identification and antimicrobial susceptibility testing: *Gram-stained smear * Methanol fixation of the smear preserves bacterial and cellular morphology -To avoid false-positive results: (flagged by the instrument as positive but Gram stain-negative) *Acridine orange (AO) staining (Particularly helpful in the early detection of candidemia) -Subculture positive cultures onto agar plates to isolate colonies (If bottle appears positive but no organism is seen under microscopic examination) *Media selection and culturing conditions that supports the growth of the bacteria *Resubculture for isolated colonies if polymicrobial infection is suspected *Examples of chosen media: *Chocolate agar, 5% sheep blood agar *Anaerobic blood agar *Specialized chromogenic agar for specific pathogenic organisms such as yeast (Candida spp.) and MRSA *MacConkey agar if Gram-negative bacteria are suspected Interpretation of Blood Culture Results  To distinguish probable pathogens from contaminants: -Probable contaminant *Growth of skin flora: Bacillus spp. (except Bacillus anthracis), Corynebacterium spp., Propionibacterium acnes, or coagulasenegative staphylococci in one of several cultures *Growth of multiple organisms from one of several cultures *Clinical presentation is not consistent with sepsis (physician-based, not laboratory-based criteria) *The organism causing the infection at a primary site of infection is not the same as that isolated from the blood culture -Probable pathogen *Growth of the same organism in repeated cultures (obtained either at different times or from different anatomic sites) *Growth of the organism that is suspected to cause the disease *Growth of certain organisms such as members of Enterobacteriaceae, Streptococcus pneumoniae, gram-negative anaerobes, Streptococcus pyogenes * Growth of the same organism in blood and from a sterile body site Special considerations for some organisms isolated from blood  Some microorganisms that cause bloodstream infections Do Not Grow on Artificial Media: -Diagnosed by alternative methods such as serology or molecular amplification assays  Examples of special cultures/culturing conditions: -Brucella spp (prolonged incubation period from 5-60 days) -Campylobacter (Subculture must be microaerophilic on CAMPY agar) -0 (grows 1-3 cm below surface in 2 week; failure to grow in conventional culture systems; multiple cultures; Fletcher’s medium) -Vitamin B6-dependent Streptococci: *Known as “nutritionally variant” or “satelliting” streptococci: unable to multiply without addition of 0.001% pyridoxal hydrochloride (thiol or Vitamin B6) *Sub-culturing the broth to a 5% sheep blood agar plate and either overlaying a streak of Staphylococcus aureus or dropping a pyridoxal disk to produce the supplement *Colonies of the streptococci grow as tiny satellites next to the streak  Notify physician immediately of positive culture results, gram stain smear results, other tests performed * Preliminary report after 48 hours, final report to follow (document report: name, date, time)  Treatment: -Antimicrobial therapy: *Empiric Initially * Switch to narrow spectrum after biochemical identification and susceptibility testing is completed