Staphylococcus aureus bacteremia

Staphylococcus aureus bacteremia (SAB) is still associated with a high mortality, and knowledge on risk factors and the clinical and the therapeutic aspects of SAB is still limited. This thesis focuses on the clinical aspects of SAB and its metastatic infections. In a study of all patients with bacteremia in Copenhagen County October 1992 through April 1993 (study I) we emphasized previous findings, that S. aureus is one of the most frequent pathogens in bacteremia, and in a case control study also in Copenhagen County 1994-95 (study II) we demonstrated, that not only an inserted central venous catheter and nasal S. aureus carriage but also hyponatremia and anemia are important risk factors for hospital-acquired SAB (study II). Studies on the treatment of SAB have pointed out, that the eradication of a primary is important, but there are only limited clinical studies dealing with antibiotic treatment. By logistic regression analysis, we were able to demonstrate that focus eradication is essential, but also that treatment with dicloxacillin 1 g x 4 or 2 g x 3 are superior to 1 g x 3 (studie III), indicating that the time for serum concentration above the Minimal Inhibitory Concentration (MIC) for the bacteria plays a role in the outcome of SAB treatment. S. aureus osteomyelitis secondary to SAB is frequently observed. No other countries, however, have a centralized registration, which make it possible to evaluate a large number of these patients. Since 1960, The Staphylococcal Laboratory, Statens Serum Institut in Copenhagen, has registrated selected clinical informations from nearly all patients with positive blood cultures of S. aureus. Based on this registration, we were able to show an increased number of S. aureus osteomyelitis among older patients and a decreased number of S. aureus osteomyelitis of femur and tibia among younger infants in the period 1980-90 (study IV). By reviewing the records of a large number of patients with vertebral S. aureus osteomyelitis, we could evaluate important aspects in the diagnosis and treatment of these patients (study V). We illustrated, that symptoms and laboratory findings were relatively unspecific, and CT-scanning or bone scintigraphy were absolutely necessary for the diagnosis (study V). The relatively high number of patients in the study allowed us to evaluate different treatment regimens, and we found, that treatment with penicillinase-stable penicillins four grams daily for at least eight weeks was necessary (study V). S. aureus meningitis is relatively uncommon and most often a neurosurgical infection based on the presence of a catheter. Meningitis secondary to SAB is relatively rare. The nationwide registration on Statens Serum Institut enabled us to study a large number of patients with special emphasis on clinical, outcome and treatment (study VI). We found, that these patients often were older people with chronic underlying diseases, the infection developed as a community-acquired infection, and the patients also had an unknown focus of infection. Furthermore, these patients often had other secondary manifestations such as endocarditis or osteomyelitis and an extremely high mortality (study VI). Finally, I believe that our studies will contribute to reduce the incidence of SAB and improve the diagnosis and treatment of SAB in the future.     

Recurrent Staphylococcus aureus bacteremia.

Sequential blood isolates from eight patients with 10 episodes of recurrent Staphylococcus aureus bacteremia were typed by restriction endonuclease analysis of plasmid DNA (REAP DNA fingerprinting) and immunoblotting. There were six early recurrences (within 2 months of stopping antimicrobial therapy) and four late recurrences. All early recurrences isolates were identical to initial isolates. These recurrences were defined as possible relapses. Three of four late recurrence isolates were different from the preceding isolates recovered from four patients. This was considered indicative of new infections. There was complete concordance between REAP DNA fingerprinting and immunoblot typing results. However, four isolates lacked plasmid DNA and could be typed only by immunoblotting. All initial isolates from different patients were different types by immunoblotting and by REAP DNA fingerprinting (except for those lacking plasmid DNA). The bacterial traits detected by these methods appear to be stable in vivo for up to 3 months. Relapsing infections were associated with the presence of intravascular foreign bodies and vancomycin therapy of the preceding episodes.     

Hemagglutination by Staphylococcus aureus strains responsible for human bacteremia or bovine mastitis

Although hemagglutination by Staphylococcus aureus has been associated with the pathogenesis of bovine mastitis, this trait has not been characterized with regard to human disease. In this study, the prevalence of hemagglutination in 100 strains of S. aureus responsible for bovine mastitis or human bacteremia, was characterized. Under optimum conditions hemagglutination was noted in 23% of the bovine strains, but only 13% of human strains, leading us to conclude that this trait is not a significant virulence determinant in human systemic infection. Additional studies indicate the hemagglutinin of S. aureus strains responsible for human bacteremia is proteinaceous in character.     

Characterization of Staphylococcus aureus strains isolated from humans in Argentina

Humans are a natural reservoir of Staphylococcus aureus and asymptomatic colonization is far more common than infection. The aim of this work was to characterize genotypically 68 S. aureus strains isolated from nasal swabs of healthy people and from human clinical infections. A total of fourteen (20%) strains were susceptible to all the antimicrobials tested. The strains isolated from nasal swabs showed the lowest percentages of resistance. Resistance to one or more than one antibiotics tested was detected in 83% and 70% of the S. aureus strains isolated from clinical infections and nasal swabs, respectively. All of the 68 S. aureus strains were subject to RAPD-PCR analysis. Cluster A-I grouped 42 (87%) clinical infection strains and cluster A-II grouped 13 (65%) strains isolated from nasal swabs suggesting a genetic relationship among S. aureus strains. Cluster A-II grouped 65% of the S. aureus strains associated with the anterior nares, suggesting that these strains may be adapted to this site. Furthermore, five RAPD profiles isolated from nasal swabs, belonged to clusters B to F, were similar to strains isolated from clinical infection, suggesting that they might have a high propensity to cause disease. The results of the present study allow a characterization of S. aureus strains isolated from humans and shows that some S. aureus genotypes from nasal swabs are similar to the genotypes obtained from clinical infections, suggesting that clinical isolates may be originated from human normal flora.     

Ecology of Staphylococcus aureus: Characterization of strains from chicken

The majority of S. aureus strains isolated from beak-swabs and pathological processes in chicken shows coagulation of human plasma (not of bovine plasma), crystal violet-type A, hemolysine-type A, formation of fibrinolysin, not formation of DNase and reactions with the experimental phage A1591. Because of the absence of DNase-formation and the reaction-specificity for phage A1591 we propose to designate these strains as host-specific variety gallinae of S. aureus. The strains from chicken are compared with strains of human, bovine, and ovine origin. An ecological study in a chicken farm has shown that S. aureus strains from chicken are not found in man and vice versa.     

Epidemiology of community-acquired Staphylococcus aureus bacteremia.

Methicillin-resistant Staphylococcus aureus (MRSA) is an important nosocomial pathogen which has been isolated with increasing frequency in recent decades. Community-acquired MRSA (CA-MRSA) infections have also become increasingly important in recent years. This study retrospectively analyzed the risk factors, duration of hospitalization, yearly trend and seasonal variation in prevalence, and antibiotic susceptibility of isolates of community-acquired S. aureus (CASA) bacteremia and CA-MRSA bacteremia from patients treated in a teaching hospital in northern Taiwan. A total of 104 clinical isolates of CASA bacteremia were collected between January 1999 and December 2001. Among these, 35 (33.7%) were identified as MRSA. After multivariate analysis, the independent risk factors for developing CA-MRSA bacteremia were diabetes mellitus (p=0.028), chronic obstructive lung disease (p=0.037), and renal insufficiency (p=0.041). Only 6 (17.1%) patients in the MRSA group had no identified risk factors. Most of the isolates of CA-MRSA had a high degree of resistance to most antibiotics, including clindamycin (71.4%), trimethoprim-sulfamethoxazole (65.7%), and chloramphenicol (41.2%). No major trend or seasonal variation in the prevalence was found during the study period. No difference in mortality related to resistance pattern was found. Although CA-MRSA is not the major pathogen in community-acquired bacteremia, it should be included in the differential diagnosis of Gram-positive bacterial bloodstream infection, especially in those patients with risk factors. Early empiric therapy with glycopeptides in these patients may reduce morbidity and mortality.     

Characterization of clinical strains of Staphylococcus aureus associated with pneumonia.

A total of 5 Staphylococcus aureus strains from patients with postinfluenzal staphylococcal pneumonia, 7 from burn patients with staphylococcal pneumonia, and 21 from the nasopharynx of carriers were phenotypically characterized. All or most strains produced coagulase, clumping factor, DNase, thermostable DNase, protease, gelatinase, lipase, and pigment; the strains were low to moderate producers of extracellular protein A, fibrinolysin, and alpha-hemolysin. All strains were sensitive to mercury, half were sensitive to arsenate and cadmium, and 67 to 92% were resistant to penicillin. Differences between strains were not statistically significant. Cell surface hydrophobicity was determined by measuring percent adsorption to hexadecane. Hydrophobicity of postinfluenzal staphylococcal pneumonia strains was significantly lower than that of pneumonia strains from burn patients and carriers (P less than 0.005). Immunoblot experiments with sera immune to one clinical test strain allowed the separation of all strains into three groups based on probe-positive reactions with primarily four staphylococcal polypeptides (154,200, 130,000, 77,100, and 64,400 molecular weight). The difference in distribution of clinical and carrier strains was highly significant (P = 0.007).     

Characterization of Staphylococcus aureus strains in a rabbit model of osseointegrated pin infections

Staphylococcus aureus is a common infecting agent of many surgical sites. As a commensal organism to humans and rabbits, the infection process may occur due to native or exogenous S. aureus. We applied exogenous S. aureus ATCC 49230 once weekly to the surgical site of an osseointegrated pin in 20 New Zealand white rabbits. Clinical signs of infection resulted in euthanasia and at necropsy samples were collected from putatively infected sites. The predominant organism cultured was S. aureus. We observed various beta-hemolysis patterns of S. aureus on culture media and used pulsed field gel electrophoresis (PFGE) to determine whether there were distinct strains of S. aureus collected from various sites of the rabbits. On the basis of PFGE results, we found that the exogenous S. aureus ATCC 49230 was not the S. aureus cultured during necropsy, but that S. aureus native to the rabbits was in fact the infecting agent. We conclude that this rabbit model for S. aureus infection, which has not been described previously, may contribute to understanding the pathogenesis of S. aureus infections in future studies with simulated osseointegrated pin infections secondary to S. aureus.     

Emergence of a clinical daptomycin-resistant Staphylococcus aureus isolate during treatment of methicillin-resistant Staphylococcus aureus bacteremia and osteomyelitis

The emergence of a clinically daptomycin-resistant Staphylococcus aureus isolate occurred during treatment of methicillin-resistant S. aureus bacteremia and probable vertebral osteomyelitis. The breakthrough isolate was indistinguishable from pretreatment daptomycin-susceptible isolates by pulsed-field gel electrophoresis. Daptomycin nonsusceptibility was confirmed by MIC and time-kill curve analyses.     

Lysogenicity of methicillin-resistant strains of Staphylococcus aureus

The lysogenic status of 23 strains of methicillin-resistant Staphylococcus aureus, isolated at the Royal Prince Alfred Hospital, Sydney, since 1980, was studied. Twenty strains, belonging to the four predominant phage types isolated in this hospital, carried the same lysogenic phage which we have designated C. Three other phages were isolated from five strains belonging to phage type 84/85/90. The presence of phage C had little effect on the phage-typing pattern of the strains. Similarly, lysogenization with the other three phages did not result in a significant change in phage-typing patterns. However, when strain 1489, isolated in 1969, was lysogenized with these three phages, there was a change in phage-typing pattern. Lysogenization of this strain with phage 47T resulted in a marked loss of sensitivity to both group-I and group-III phages. The lysogenic status of these methicillin-resistant strains of S. aureus was compared with that of strains isolated between 1967 and 1970. There was no evidence that the strains isolated recently were either related to, or derived from, the earlier ones.     

Epidemiology of Staphylococcus aureus bacteremia in Denmark from 1957 to 1990

Objective: To investigate the changes in epidemiology of Staphylococcus aureus (SA) bacteremia in Denmark over a 30-year period, where the population has remained stable.
Method: Bacteriologic and clinical data were generated on 17 712 SA strains from virtually all SA bacteremia cases in Denmark from 1957 to 1990 submitted to our laboratory for phage typing. The data were related to information about population, hospital activity and blood-culturing activity during that period.
Results: SA bacteremia cases increased from 3 to 20/100 000 inhabitants per year, with the largest increases in incidence rates for the <1-year and>50-year age groups. While blood-culturing activity increased three-fold during the period, the rate of SA bacteremias actually decreased relative to the number of blood cultures taken. The increase in SA bacteremia cases was mainly due to increases in nosocomial infections for all age groups and was related to the increasing admission rates to Danish hospitals. Major shifts in antibiotic resistance patterns and phage types took place during the period, i.e. a marked reduction in multiresistant (including methicillin-resistant) strains, but could not explain the change in the epidemiology of the infections.
Conclusions: The data indicate that increases in SA bacteremia rates correlated significantly with increasing numbers of admissions to hospitals. The main increase in SA bacteremia rates was represented by nosocomial infection, although increasing blood-culturing activity during the period may have contributed.     

Retrospective study of outcome in patients treated for Staphylococcus aureus bacteremia

Objective: To investigate whether a change in current treatment practice for Staphylococcus aureus bacteremia from flucloxacillin and aminoglycoside to flucloxacillin and fusidic acid was associated with any changes in outcome.
Method: A retrospective analysis was carried out of 316 episodes of S. aureus bacteremia diagnosed and treated in a tertiary hospital complex between 1983 and 1993. Outcomes considered were (1) death related to the infection and (2) relapse following cessation of antibiotic therapy.
Results: Mortality related to infection, which occurred in 24% of patients, was unrelated to treatment with the combination of flucloxacillin and fusidic acid; however, increasing age was a significant risk factor (OR per decade = 1.35, 95% CI = 1.18-1.55), and increasing duration of treatment (OR per week of treatment = 0.63, 95% CI = 0.52-0.77), use of flucloxacillin (OR = 0.30, 95% CI = 0.14-0.64), presence of an intravascular device (OR = 0.39,95% CI = 0.20-0.78) and presence of a skin lesion (OR = 0.51, 95% CI = 0.26-0.99) were significant protective factors. The only factor significantly related to relapse, which occurred in 11% of patients, was treatment with the combination of flucloxacillin and fusidic acid (OR = 0.32, 95% CI = 0.12-0.85). There was approximately a 70% reduction in the risk of relapse if this combination was used.
Conclusions: This retrospective analysis suggests a clinically important protective effect of fusidic acid against relapse in patients with S. aureus bacteremia. Although the results were adjusted for potential confounding factors, the possibility of bias remains. There is a need for a prospective randomized trial to evaluate the effectiveness of flucloxacillin and fusidic acid for treating S. aureus bacteremia.     

Clinical significance of Staphylococcus aureus bacteremia in patients with liver cirrhosis

Patients with liver cirrhosis (LC) have impaired immunity and thus are predisposed to infections. Few studies have attempted to evaluate Staphylococcus aureus bacteremia (SAB) in LC patients. Therefore, this study prospectively evaluated the clinical characteristics and outcomes of 642 episodes of SAB from August 1, 2008 to September 31, 2010. Of 642 patients with SAB, 109 (17.0?%) were classified as LC patients whereas the remaining 533 (83.0?%) were classified as non-LC patients. The 30-day mortality rate of LC patients was significantly higher than that of patients with other diseases (32?% vs. 22?%, respectively; P?=?0.047). The 30-day mortality rates of patients with MSSA bacteremia and MRSA bacteremia were not significantly different among LC patients (35.1?% with MSSA vs. 26.9?% with MRSA; P?=?0.41). A univariate analysis of the 30-day mortality rate of LC patients with SAB for survivors and non-survivors showed that rapidly fatal or ultimately fatal according to the criteria of McCabe and Jackson (OR 5.0; 95?% CI 1.60–15.65), septic shock at initial presentation (OR 3.5; 95?% CI 1.18–10.39) and Child-Pugh class C (OR 2.8; 95?% CI 1.20–6.59) were associated with increased mortality. In contrast, the removal of the eradicable focus was associated with decreased mortality (OR 0.14; 95?% CI 0.04–0.52). Disease severity and liver dysfunction may be useful for predicting the prognosis of SAB in LC patients.     

Staphylococcus aureus bacteremia in patients with hematological malignancies and/or agranulocytosis

A total of 6,253 cases of Staphylococcus aureus bacteremia, including 274 (4.4%) endocarditis cases, were registered in Denmark in the period 1975-1984. Patients with hematological malignancies and/or agranulocytosis accounted for 479 of the bacteremia cases. The incidence of endocarditis in this group of patients was only 0.4% as compared to 4.7% in other patients with staphylococcal bacteremia (p less than 0.01). The lower incidence of endocarditis complicating bacteremia in these patients may justify a shorter course of therapy than usually recommended for suspected endocarditis. Patients with hematological malignancies and other patients with agranulocytosis had a higher mortality (49 and 46%, respectively) than other patients with S. aureus bacteremia (33%). The highest mortality was found in patients with multiple myeloma (71%, p less than 0.01), the lowest in patients with acute lymphocytic leukemia (28%, p less than 0.01). The higher mortality in these patients may indicate that empiric antibiotic regimens in granulocytopenic patients should include a specific anti-staphylococcal agent.     

Characterization of Staphylococcus aureus enterotoxin L

Staphylococcus aureus causes a wide variety of diseases. Major virulence factors of this organism include enterotoxins (SEs) that cause both food poisoning and toxic shock syndrome. Recently, a novel SE, tentatively designated SEL, was identified in a pathogenicity island from a bovine mastitis isolate. The toxin had a molecular weight of 26,000 and an isoelectric point of 8.5. Recombinant SEL shared many biological activities with SEs, including superantigenicity, pyrogenicity, enhancement of endotoxin shock, and lethality in rabbits when administered in subcutaneous miniosmotic pumps, but the protein lacked emetic activity. T cells bearing the T-cell receptor beta chain variable regions 5.1, 5.2, 6.7, 16, and 22 were significantly stimulated by recombinant SEL.     

Molecular Characterization of Endocarditis-Associated Staphylococcus aureus

Infective endocarditis (IE) is a life-threatening infection of the heart endothelium and valves. Staphylococcus aureus is a predominant cause of severe IE and is frequently associated with infections in health care settings and device-related infections. Multilocus sequence typing (MLST), spa typing, and virulence gene microarrays are frequently used to classify S. aureus clinical isolates. This study examined the utility of these typing tools to investigate S. aureus epidemiology associated with IE. Ninety-seven S. aureus isolates were collected from patients diagnosed with (i) IE, (ii) bloodstream infection related to medical devices, (iii) bloodstream infection not related to medical devices, and (iv) skin or soft-tissue infections. The MLST clonal complex (CC) for each isolate was determined and compared to the CCs of members of the S. aureus population by eBURST analysis. The spa type of all isolates was also determined. A null model was used to determine correlations of IE with CC and spa type. DNA microarray analysis was performed, and a permutational analysis of multivariate variance (PERMANOVA) and principal coordinates analysis were conducted to identify genotypic differences between IE and non-IE strains. CC12, CC20, and spa type t160 were significantly associated with IE S. aureus. A subset of virulence-associated genes and alleles, including genes encoding staphylococcal superantigen-like proteins, fibrinogen-binding protein, and a leukocidin subunit, also significantly correlated with IE isolates. MLST, spa typing, and microarray analysis are promising tools for monitoring S. aureus epidemiology associated with IE. Further research to determine a role for the S. aureus IE-associated virulence genes identified in this study is warranted.