Lecture notes, lectures 11 - Micro module 1 - CVS PDF

Preview

Summary

Download Lecture notes, lectures 11 - Micro module 1 - CVS PDF

Description

Laboratory #5: Cardiovascular Infections Introduction The cardiovascular system consists of the heart, blood vessels and blood. As blood circulates throughout the body, it comes into contact with many tissues and organs. As such, the bloodstream serves as an excellent vehicle for the spread of pathogenic organisms – facilitating the infection of multiple organ systems. Bloodstream infections manifest clinically into many forms. For example, persistent bacteremia (presence of bacteria in the blood) is suggestive of intravascular infections such as endocarditis or catheter-related infections. These types of infections are life-threatening, and the appropriate diagnosis and management of such infections are critical to patient survival. In lecture, you learned that cardiovascular infections can manifest in the heart valves, myocardium, and pericardium. Moreover, you have learned that each site of infection possesses characteristic signs and symptoms that, when identified, help facilitate diagnosis. In this laboratory module, you will explore some of the specimen collection methods and laboratory diagnostic techniques used to identify acute bacterial endocarditis; a serious cardiovascular infection. Patient Scenario: Anne is a 33 year old woman who presents to the emergency room with a fever, shortness of breath, and chest pain. In the emergency room, Anne’s vital signs are as follows: blood pressure 100/70, heart rate 110, temperature 39°C, and a respiratory rate 33 with an oxygen saturation level of 90% on room air. Anne is examined by the emergency room nurse who finds injection marks on both arms. Anne tells the nurse that she has been abusing heroin for the past 3 years, and that she last injected drugs 3 days ago. Examination of Anne’s chest reveals a dullness too percussion in the left lung base and crackles and wheezes in the same area. A loud heart murmur is also heard. Anne tells the emergency room physician that this is the first time she has ever been told about having a heart murmur. Based on the physical exam, a chest x-ray and 2 sets of blood culture are ordered. Anne’s chest x-ray reveals several small areas of infiltrates in the lower lobe of her left lung. Within 4 hours of delivering the blood samples to the medical microbiology lab, the laboratory technician calls to report that both sets of blood cultures are positive for Staphylococcus aureus. Staphylococcus aureus is a gram positive bacterium that appears microscopically as grape-like clusters. Staphylcocci are ubiquitous, and can survive extreme conditions of drying, heat, and low-oxygen and high-salt environments. S. aureus has many surface proteins that allow the organism to bind to tissues and foreign coated with fibronectin, fibrinogen, and collagen, thereby allowing the bacterium to adhere to sutures, catheters, prosthetic valves and other devices such as dirty needles. S. aureus colonizes the skin and mucous membranes of 30-50% of healthy adults and children. Rates of carriage on the hands of other skin areas of more than 50% of these individuals occur in children with burns, and in people with frequent needle use (e.g. diabetes mellitus, hemodialysis, recreational drug use, allergy shots). As such, these individuals (including i.v. drug users such as Anne) are at an increased risk of vascular-access bacteremia. At this point in the patient scenario, we have learned the following information: -

Anne has a fever

-

Anne is experiencing chest pain and an increased heart rate Anne has low blood pressure Anne is experiencing shortness of breath and an increased respiratory rate Anne’s chest x-ray reveals lower lobe infiltrates in the left lung Anne has a new heart murmur Anne’s blood cultures are positive for S. aureus Anne is an IV drug user

Based on this information, what diagnosis do you suspect? -

Acute bacterial endocarditis Cellulitis Community acquired pneumonia

Intravenous drug users are at an increased risk of vascular-access bacteremia, which can then lead to endocarditis. Anne’s new and loud heart murmur is consistent with heart valve damage. -

Endocarditis: infection and inflammation of the heart valves and endocardium

Right-sided endocarditis most often occurs when an aggressive species of skin bacteria, such as S. aureus, enters the bloodstream and attacks a normal undamaged heart valve. When S. aureus bacteria begin to multiply inside the heart, small clumps of bacteria called septic emboli may move into the bloodstream – spreading the infection to other organs, especially the kidneys, lungs and brain. It is likely that the pneumonia seen in Anne’s chest x-ray is a result of bacteria from her heart moving into, and thus infecting, her lungs. Endocarditis develops as bacteria attach to the surface of the heart valve and multiply, causing damage that promotes the formation of fibrin-platelet vegetations. These thick vegetations allow bacteria to grow protected from host defences. Symptoms of endocarditis include fever and a heart murmur (an indication of poor valve function). Incorrect answer: Community Acquired Pneumonia While the infiltrates observed in Anne’s lungs suggest she has pneumonia, it is unlikely that the lungs represent the primary site of infection. It is not uncommon for individuals with pneumonia to become bacteremic , however Staphylococcus aureus rarely causes community acquired pneumonia (as you learned in lecture Streptococcus pneumonia and Chlamydia pneumonia are the most common causes of CAP in Canada). As such, it is likely that Anne’s pneumonia is not community acquired, but rather, a complication of an infection elsewhere in the body. Anne’s new heart murmur and intravenous drug use suggests a cardiovascular infection. -

Intravenous drug use: Intravenous drug users are at a very high risk for acute endocarditis, since their addiction allows aggressive S. aureus bacteria many opportunities to enter the blood through broken skin. Dirty drug paraphernalia increases this risk. If untreated, this form of endocarditis can be fatal in less than 6 weeks

Incorrect Answer: Cellulitis While cellulitis is associated with bacteremia, and can be caused by Staphylococcus aureus, it is unlikely that Anne has this type of infection. If Anne did have cellulitis, we would expect to find a hot, red, and swollen lesion on Anne’s body during the physical exam. However, intravenous drug users are at an increased risk of skin and soft

tissue infections. If Anne chooses to continue to abuse drugs, it is important that Anne use a new needle when injecting, and cleans her skin properly before each injection. Based on Anne’s history, signs and symptoms, and recent laboratory results, the physician makes a diagnosis of acute bacterial endocarditis. Anne is admitted to hospital and started on two antibiotics intravenously: -

Oxacillin: a beta-lactam antibiotic in the penicillin class. It is highly effective in the treatment of Staphylococcus aureus Vancomycin: is a glycopeptides antibiotic indicated for the treatment of serious, life-threatening infections by Gram-positive bacteria that are unresponsive to other less toxic antibiotics

The following day, the laboratory reports that methicillin-resistant Staphlococcus aureus (MRSA) has been isolated from Anne’s blood cultures. As a result, the oxacilin is stopped and Anne is continued on vancomycin. Infection Control is notified, and Anne is placed into contact precautions. Methicillin-resistant Staphylococcus aureus (MRSA) is a strain of Staphylococcus aureus that is resistant to betalactam antibiotics. This resistance is caused by an alteration in the penicillin binding protein that is carried on the mecA gene. Because of this mutation, this organism is resistant to penicillin, ampicillin, piperacillin, cloxacillin, all cephalosporis, meropenem and all beta-lactam/beta-lactamase inhibitor combinations. -

A new strain of community-associated MRSA has been seen in Canada. This strain contains a gene called the Panton-Valentine-Leukocidiin gene (PVL gene) which allows the organism to penetrate intact skin and cause more severe infection. We have see bloodstream infections with this organism in Ontario, Albert, and BC. Therefore, most life-threatening S. aureus infections should be treated with a drug like vancomycin until the sensitivity of the organism is known (In Anne’s case, it is an MRSA infection)

Consequently, Staphylococcus aureus can only be treated with a few agents (vancomycin, trimethoprimsulfamethoxoazole, linezolid and a few others). Patients can be colonized with this organisms (e.g. it can live harmlessly on the skin, or in the nose or anus) or it can cause disease, as in Anne’s case. MRSA: Infection Control: In the hospital setting, S. aureus can easily spread from one patient to another unless proper precautions are used. Infection Control measures for MRSA include 1) isolating the infected patient, and 2) implementing the following contact precautions: -

Use of gloves when entering patient room & changing them when they are soiled Use of good hand washing technique after removing gloves Use of gowns when entering the patient room Dedication of patient equipment (when possible)

Some facilities also use masks to reduce transmission – masks prevent patients and health care workers from touching their noses, thereby reducing the risk of MRSA colonizing the nasal mucous of healthcare providers. However, this is not a consistent practice. It is important to remember that MRSA is not transmitted by the airborne route. Contact precautions are necessary when treating a patient with MRSA because 1) MRSA is highly resistant to antibiotics; 2) there are limited treatment options for MRSA; 3) MRSA is most often transmitted from one patient to another via healthcare workers, 4) MRSA infections can be fatal, and 5) MRSA infections can be prevented by using proper control methods to reduce transmission.

Inside the Medical Microbiology Laboratory: Cardiovascular Infections: Specimen Collection and Laboratory Diagnosis. Blood specimens are a key specimen for the diagnosis of acute bacterial endocarditis. Blood specimens are collected as a set: one aerobic bottle (green) and one anaerobic bottle (pink). In adults, 10-20ml of blood is needed per set. The bottles shown in the previous slide have a rubber stopper covering the opening – a needle can be inserted through this rubber to inoculate the blood into the bottle. Each bottle contains 1) a nutrient broth to promote the growth of bacteria and, 2) binding agents that remove antibiotics from the blood specimen. The bottom of each bottle contains a semi-permeable membrane which allows CO2 to diffuse from the specimen, causing a colour change at the bottom of the bottle. CO2 production is an indicator of bacterial growth. If the bottom of the bottle changes colour, a machine in the laboratory detects this change and alerts the technician that bacteria is growing in the blood specimen. The workup of the specimen (gram stain, blood culture) can then proceed. The following steps are involved in collecting a blood specimen: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Assess the client for bleeding risks and ensure no contraindications exist Explain the procedure to patient Wash your hands thoroughly Position patient Apply disposable gloves Clean rubber stoppers on top of the blood culture bottles with disinfectant Apply tourniquet Ask client top open and close their fist several times Inspect for best venipuncture site Palpate vein Clean venipuncture site with antiseptic (containing chlorhexidine) Insert needle with bevel side up Hold needle in position once inserted into the vein Collect required amount of blood: 10-20 ml per set for adults; 3-5 ml per set for a child; 2-3 per set for an infant Release tourniquet once blood has been collected Apply gauze or alcohol pad over needle Remove needle Apply pressure to venipuncture site Dispose of needle safely Monitor to ensure site stops bleeding

Question: Since the blood is usually a sterile specimen, can bacteria cultured from a blood specimen ever be considered contaminants of the sample? YES, bacteria cultured from a blood specimen could be a contaminant – not all bacteria grown in culture are the cause of infection. From the skin and soft tissue infection lecture, you should recall that the skin has a variety of bacteria growing on its surface (normal skin flora). If blood is not collected using aseptic technique, bacteria from the skin can get into the blood culture bottles.

Growing a typical skin bacteria (e.g. Coagulase negative staphylococci) from only one bottle (or one set of cultures) suggests that skin contaminants were introduced during blood collection. For this reason, at least two sets of blood specimens are always ordered.

No, all bacteria cultured from a blood specimen are the cause of infection. Incorrect. During blood collection, contaminants (i.e. bacteria) can enter into the specimen. These bacteria are not response for the infection, and are therefore considered contaminants of the sample. How contaminants introduced into a blood specimen? Try and identify the steps involved in collecting a blood culture would be susceptible to contamination.

3. Wash your hands thoroughly 5. Apply disposable gloves 6. Clean rubber stoppers on top of blood culture bottles with disinfectant 10. Palpate vein 11. Clean venipuncture site with antiseptic (containing chlorhexine) 12. Insert needle with bevel side up 13. Hold needle in position once inserted into the vein 16. Apply gauze or alcohol pad over needle Gram stains are reported out to the clinical areas. The positive blood culture is then place onto agar plates to grow the bacteria – this allows the bacteria to be identified. On sheep’s blood agar, S. aureus grows in a round, golden-yellow coloured colonies. Beta-hemolysis is also often observed. On chocolate agar, the characteristic golden colour of S. aureus is also apparent. S. aureus is catalase positive. Patient Scenario: Once on antibiotic therapy, Anne begins to improve clinically, but continues to have ongoing fevers. Repeated blood cultures are still positive for S. aureus the following day. The persistent positive blood cultures are due to 1) the intra-cardiac source of the infection, and 2) the severity of Anne’s infection. Duration of therapy for serious S. aureus infections depends on the site and severity of infection, but usually it is 4 weeks or more (as is the case for endocarditis) before the blood is sterile. Parenteral antibiotic therapy is recommended for Anne’s entire treatment....