Emerging patterns of microbial resistance

Microbial resistance arises by mutation or by inheritance. The latter is plasmid-mediated and transferable and may erode multidrug resistance to beta-lactams, aminoglycosides, tetracyclines, macrolides, lincosamides, sulfonamides, and trimethoprim. Resistance genes may transfer from one plasmid to another or from a plasmid to the chromosome or to a bacteriophage, thereby allowing rapid dissemination of resistance among bacteria. Mutational or chromosomal resistance is not readily transferable between different bacterial species or genera but is nonetheless medically important for resistance to isoniazid, methicillin, nalidixic acid, rifampin, and expanded spectrum cephalosporins.

Initial detection of bacteremia by subculture of unvented tryptic soy broth blood culture bottles

Routine subculture of macroscopically negative blood cultures is a traditional blood culture procedure. The need to perform routine early (6-17 hr) and late (48 hr) subculture of unvented blood culture bottles when a simultaneous subculture of the vented bottle is performed has been questioned. Blood cultures in paired vented and unvented tryptic soy broth (TSB) bottles from 4574 patients were examined retrospectively. Subculture of unvented TSB bottles provided initial detection of 412 (5.0%) isolates from 277 (6.1%) patients and was comparable to that of vented TSB bottles for Pseudomonas and all other microorganisms, except for the Enterobacteriaceae (p less than 0.001; vented TSB), Candida (p less than 0.001; vented TSB), and Haemophilus influenzae (p less than 0.01; unvented TSB). Of the H. influenzae isolates, 46% were detected initially by subculture of the unvented TSB bottles; early subculture recovered 67% of these isolates. The value of subculture of unvented TSB bottles is minimized when subculture of the vented TSB bottle is routinely performed; however, routine subculture of the unvented bottle is recommended whenever TSB is used for detection of bacteremia in patients in whom H. influenzae infection is possible.

Comparison of the Roche Septi-Chek blood culture bottle with a brain heart infusion biphasic medium bottle and with a tryptic soy broth bottle

In a comparison of 1,368 positive blood cultures, a vented Roche Septi-Chek (V-RSC) blood culture bottle was superior to an unvented tryptic soy broth-containing bottle (Difco) for the recovery of all aerobic and facultatively anaerobic microorganisms. Anaerobic bacteria were recovered more frequently and earlier in the unvented tryptic soy broth-containing bottle. A separate comparison of 529 positive blood cultures was conducted to examine the performance of the V-RSC bottle with that of a vented brain heart infusion biphasic medium. The V-RSC bottle recovered significantly more isolates of Enterobacteriaceae and of anaerobic bacteria than did the vented brain heart infusion biphasic medium. The V-RSC bottle is a reliable blood culture system for all aerobic and facultatively anaerobic microorganisms. Because of its suboptimal recovery of anaerobic bacteria, it is recommended that the V-RSC bottle be used in combination with an unvented vacuum blood culture bottle.

Contamination of cultures processed with the isolator lysis-centrifugation blood culture tube

Overall contamination (on- plus off-streak) of the Isolator (Du Pont Co.) blood culture tube (23%) was greater than that of a conventional broth blood culture bottle (0.6%) or that of a biphasic blood culture bottle (1.3%). To determine the source of this contamination, Isolator cultures of blood from 59 healthy volunteers and of sterile broth from 60 vials were made. A total of 37% of the blood cultures and 22% of the broth cultures were contaminated (P = 0.06). Staphylococcus epidermidis-contaminated cultures represented 31 and 10% of the blood and broth cultures, respectively (P = 0.06). Contamination of plates processed on a bench top, in front of horizontal laminar flow, and in a biological safety cabinet with vertical laminar flow were compared. Processing plates in a biological safety cabinet resulted in a significant reduction in the number of contaminated plates (P less than 0.05). The contamination rate for 7,874 Isolator blood cultures processed in the biological safety cabinet was significantly decreased to 6.7% on-streak (9.3% on- plus off-streak). Contamination of Isolator-processed blood cultures originated from the laboratory and the patient. The former can be reduced by inoculating plates in a vertical laminar flow biological safety cabinet and by maintaining adequate quality control of media. The latter may be unavoidable.

Aerococcus viridans endocarditis

Aerococcus viridans organisms are gram-positive cocci with a strong tendency to form tetrads. These bacteria have infrequently been encountered as a human pathogen, particularly in bacterial endocarditis. A review of the literature suggests that treatment of A. viridans endocarditis should be similar to that for endocarditis caused by penicillin-susceptible streptococci.

Early detection of positive blood cultures by the acridine orange staining technique

Staining 2,205 macroscopically negative blood cultures with acridine orange after 6 to 17 h of inoculation and incubation was as sensitive as an early subculture in detecting positive blood cultures. Of the 179 positive blood cultures, 30 (16.8%) were detected by acridine orange alone, 19 (10.6%) were detected by early subculture alone, 84 (46.9%) were detected by both techniques, and 46 (25.7%) were not detected by either method. The latter group includes cultures that became positive after 48 h of incubation. Acridine orange staining of smears prepared from macroscopically negative blood cultures after 6 to 17 h is a rapid, reliable method to detect positive blood cultures.

Effects of trimethoprim-sulfamethoxazole and incubation atmosphere on isolation of group A streptococci

The effects of selective media and incubation atmosphere on the isolation of group A beta-hemolytic streptococci were evaluated. A higher percentage of group A streptococci was isolated on sheep blood agar incubated in air than in CO2 or anaerobic atmospheric conditions. Fewer non-group A beta-hemolytic streptococci were isolated on sheep blood agar incubated in air than in CO2 or anaerobically. Group A streptococcal isolation was not significantly affected by different incubation atmospheres on sheep blood agar containing trimethoprim-sulfamethoxazole, but detection time was longer than on sheep blood agar alone. No significant difference was found between isolation of group A streptococci on sheep blood agar incubated in air and that on sheep blood agar containing trimethoprim-sulfamethoxazole and incubated in 5 to 10% CO2; however, more group A streptococci were isolated on sheep blood agar in air within 24 h. Sheep blood agar incubated at 35 degrees C in air is, therefore, recommended for the isolation of group A streptococci from throat swabs.

Effects of atmosphere of incubation on recovery of bacteria and yeasts from blood cultures in Tryptic soy broth

A comparison was made of the results of blood cultures between 1974 and 1981 in unvented and transiently vented bottles of Tryptic soy broth under vacuum with CO2. A total of 14,646 isolates were available for statistical analysis. Significantly more isolates of Staphylococcus epidermidis, Staphylococcus aureus, Bacillus, Escherichia, Pseudomonas, Klebsiella, Serratia, Acinetobacter, Alcaligenes, Neisseria, and Candida were recovered from the vented bottle. Significantly more isolates of Corynebacterium, Haemophilus, Flavobacterium, Moraxella, Bacteroidaceae, and Peptostreptococcus were recovered from the unvented bottle.

Bactericidal activities of new beta-lactam antibiotics against Bacteroides fragilis

The bactericidal activities of cefoxitin, cefoperazone, cefotaxime, moxalactam, mezlocillin, and piperacillin were determined in duplicate against low and high inocula of each of 10 Bacteroides fragilis strains. Antibiotic concentrations resulting in 99.9% killing were established by quantitation of the inocula just before anaerobic incubation of tests and by subcultures of volumes sufficient to accurately determine less than or equal to 0.1% survivors. Inoculum effects on inhibitory and bactericidal activities were least for cefoxitin and greatest for cefoperazone and cefotaxime. Bactericidal-to-inhibitory concentration ratios of greater than or equal to 8 occurred most frequently with mezlocillin and piperacillin, regardless of inoculum size; however, 55% of the strains tested at the low inoculum size and 38% of those tested at the high inoculum size against the six antibiotics had bactericidal-to-inhibitory concentration ratios of greater than or equal to 4.

The importance of volume of blood cultured in the detection of bacteremia and fungemia

An analysis was made on the basis of 5,389 isolates from 5,008 positive blood cultures of the relative yields from 10, 20, and 30 ml of blood. Average yields from cultures of 20 and 30 ml of blood were, respectively, 38 and 61% greater than that from 10 ml of blood.

Microbiological and clinical evaluation of the isolator lysis-centrifugation blood culture tube

In a controlled evaluation of 6,010 blood cultures, the yield of clinically significant microorganisms was greater from a lysis-centrifugation system (Isolator, Du Pont Co.) than from a nonvented vacuum bottle containing tryptic soy broth with sodium polyanetholesulfonate and CO2 and a vented bottle containing biphasic brain heart infusion medium with sodium polyanetholesulfonate. The Isolator significantly increased the frequency of isolation of Staphylococcus aureus and Candida spp. and significantly decreased the time required for the detection of S. aureus, Pseudomonas aeruginosa, and Candida spp.; however, anaerobic bacteria were recovered significantly more frequently from nonvented bottles with tryptic soy broth, and pneumococci were recovered significantly more frequently from both bottle systems. Contamination of cultures was significantly greater with the Isolator system than with either bottle system. Regardless of the number of blood cultures obtained per septic episode, the Isolator detected microbiologically proven bacteremia or fungemia in a significantly greater number of patients and significantly decreased the time required for detection.

Group G streptococcal bacteremia: clinical study and review of the literature

Patients with group G streptococcal bactermia represented 10.8% of those with beta-hemolytic streptococcal bacteremia and 0.3% of all those with bacteremia between 1970 and 1980 at Mayo Clinic-affiliated hospitals. The most frequent portal of entry was the skin, usually in cases with preexisting edema due to previous surgical removal, irradiation, or tumor infiltration of lymph nodes, or to chronic venous insufficiency. The majority of these patients had underlying hematologic malignancies or solid tumors. Clinical response to therapy with beta-lactam antibiotics was rapid.

Discrepancies between in vitro activity of and in vivo response to antimicrobial agents

The correlation between in vitro antimicrobial activity and the in vivo response to antimicrobial therapy is affected by multiple host factors, the site and nature of the infection, and the pharmacokinetics of the antimicrobial and its penetration into areas of infection. In certain instances, discrepancies are also caused by methodologically-related variables of in vitro susceptibility tests. Examples of discrepancies between in vitro and in vivo response to antimicrobial are discussed.

Haemophilus influenzae in genitourinary tract infections

Haemophilus influenzae was isolated in pure or predominant culture from genital specimens from nine females and two males. Four of the females had vaginitis, two had IUD-related endometritis, one had an incomplete septic abortion, and one had probable urethral syndrome. Two males had urethritis.

Retrospective comparison of three blood culture media for the recovery of yeasts from clinical specimens

In a retrospective fashion, two different blood culture bottles with biphasic brain heat infusion media, one permanently and the other transiently vented, were compared to a transiently vented tryptic soy broth blood culture bottle for the recovery of yeasts from blood. A total of 95 isolates of Candida species and 19 isolates of Cryptococcus neoformans were recovered, of which 95% were from the permanently vented biphasic medium bottle incubated at 30 degrees C, 57% were from the transiently vented biphasic medium bottle incubated at 35 degrees C, and 45% were from the transiently vented tryptic soy broth bottle incubated at 35 degrees C. In 81% of the patients with candidemia, yeasts were detected either only (50%) or earlier (31%) in the permanently vented biphasic blood culture bottle. The mean recovery time was shorter in both biphasic media compared to the tryptic soy broth bottle. Cryptococcus neoformans was not recovered from the transiently vented broth, and less than 50% of the isolates of Candida glabrata and Cryptococcus neoformans grew in the transiently vented biphasic bottle. The results suggest that when fungemia is suspected, the use of a permanently vented biphasic bottle incubated at 30 degrees C will increase the recovery rate of yeasts from blood and that the transiently vented biphasic bottle and tryptic soy broth bottle are less satisfactory for this purpose.

Bactericidal and synergistic activity of moxalactam alone and in combination with gentamicin against Pseudomonas aeruginosa

The bactericidal activity of moxalactam, alone and in combination with gentamicin, was studied with macrobroth two-dimensional checkerboard and killing curve techniques against gentamicin-resistant and -susceptible strains of Pseudomonas aeruginosa. Moxalactam was bactericidal at concentrations equal to or at least two to four times its inhibitory concentrations. Synergy at clinically applicable concentrations of moxalactam and gentamicin occurred with 6 of 14 gentamicin-resistant strains and 4 of 4 gentamicin-susceptible strains by the checkerboard technique and with 7 of 14 gentamicin-resistant strains by the killing curve technique. Synergy between moxalactam and gentamicin against gentamicin-resistant strains of P. aeruginosa is unpredictable and strain- and method-dependent.

Influence of gentamicin dose size on the efficacies of combinations of gentamicin and penicillin in experimental streptomycin-resistant enterococcal endocarditis

We have evaluated the benefits of low versus high doses of gentamicin combined with procaine penicillin (1.2 X 10(6) U three times daily) in the treatment of streptomycin-resistant experimental enterococcal endocarditis in rabbits. The mean peak serum gentamicin concentration in animals treated with low-dose gentamicin (0.75 mg/kg three times daily) was 3.06 micrograms/ml (range, 2.1 to 4.2 micrograms/ml), and it was 8.05 micrograms/ml (range, 4.5 to 16.1 micrograms/ml) in animals treated with high-dose gentamicin (2 mg/kg three times daily). The mean log10 colony-forming units of enterococci per gram of cardiac valve vegetation in animals treated with procaine penicillin combined with low- or high-dose gentamicin were 2.4 +/- 1.2 and 1.4 +/- 1.3, respectively (P = not significant) after 3 days of treatment and 1.7 +/- 1.2 and 1.7 +/- 1.5, respectively (P = not significant), after 5 days of therapy. The median peak serum bactericidal titer was 1:8 in animals treated with low- or high-dose gentamicin. We detected no significant difference between low- or high-dose gentamicin combined with procaine penicillin in the efficacy of treatment of streptomycin-resistant experimental enterococcal endocarditis.

In vitro activity of moxalactam against anaerobic bacteria

Moxalactam was tested against 80 isolates of anaerobic bacteria from clinical specimens. The results of this study and a review of other studies, in which comparable susceptibility testing methods were used, demonstrated the susceptibility to moxalactam of members of the Bacteroides fragilis group, Bacteroides melaninogenicus, Fusobacterium species, anaerobic gram-positive cocci, and Clostridium perfringens.

Antimicrobial susceptibilities of nutritionally variant streptococci

Seventeen strains of nutritionally variant streptococci were examined for susceptibility to 15 antimicrobial agents. All strains grew in blood culture medium containing blood and blood products or, in the absence of blood or blood products, in broth containing glucose and pyridoxal hydrochloride. Growth was enhanced by incubation in an atmosphere containing 5%-10% CO2 or, in some cases, was CO2-dependent. Rifampin, penicillin, clindamycin, and erythromycin were the most active antibiotics tested with minimal bactericidal concentrations of 2, 1, 2, and 2 microgram/ml, respectively, for 90% of the strains; the corresponding values for vancomycin and streptomycin were 32 and 16 microgram/ml, respectively.

Lack of in vivo and in vitro bactericidal activity of N-formimidoyl thienamycin against enterococci

The minimal bactericidal concentrations of N-formimidoyl thienamycin (N-f-thienamycin) against 21 strains of enterococci isolated from patients with infective endocarditis were determined by macro- and microdilution methods. By a macrodilution technique with the minimal bactericidal concentration defined as greater than or equal to 99.9% killing of an initial inoculum, all 21 strains of enterococci were found to have minimal bactericidal concentration/minimal inhibitory concentration ratios of greater than or equal to 32. The mean minimal inhibitory concentration was 1.5 micrograms/ml (range, 0.5 to 4 micrograms/ml), and the minimal bactericidal concentration was greater than or equal to 128 micrograms/ml. The disparity between the results of our study and those published elsewhere, which reported that N-f-thienamycin is bactericidal in vitro against enterococci, may represent the relative insensitivity of the microdilution method in determining greater than or equal to 99.9% killing. The lack of in vitro bactericidal activity of N-f-thienamycin against enterococci was confirmed in vivo in the rabbit model of experimental endocarditis. N-f-Thienamycin was no more effective than penicillin alone in the treatment of experimental enterococcal endocarditis and was less effective than the combination of penicillin and gentamicin. The results indicate that N-f-thienamycin should not be used alone in the treatment of enterococcal endocarditis.

Penetration of cefazolin into normal and osteomyelitic canine cortical bone

The ability of cefazolin to cross the capillary membrane and its concentrations in the interstitial fluid spaces were studied in normal and osteomyelitic canine bone. The maximum extraction after a single capillary passage and the net extraction after 3 min, determined with triple-tracer indicator-dilution techniques, demonstrated that cefazolin readily traversed the capillaries of normal and osteomyelitic bone. These studies suggest that the altered pathophysiology of osteomyelitic tissue and the complex diffusional characteristics of cefazolin enhanced the ability of this agent to cross the endothelial cells lining the capillaries of osteomyelitic bone. Volume of distribution studies demonstrated that cefazolin was distributed in the plasma and interstitial fluid spaces of normal cortical bone. Although these spaces were increased 330 and 941% in osteomyelitic tissue, the distribution of cefazolin increased proportionally. There was a direct correlation between the calculated concentrations of cefazolin in the interstitial fluid spaces of normal and osteomyelitic cortical bone and the simultaneous serum levels in animals in which a steady-state equilibrium had been achieved. These studies suggest that a physiological barrier or concentration gradient for cefazolin does not exist in normal or osteomyelitic bone. Cefazolin can cross the capillary membranes of bone and achieve bactericidal concentrations in the interstitial fluid space of normal and osteomyelitic tissue.

The antimicrobial removal device. A microbiological and clinical evaluation

A parallel study of blood cultured conventionally and following processing in an Antimicrobial Removal Device (ARD) was conducted with patients suspected of being clinically bacteremic. Cultures yielded 205 isolates from 87 clinically bacteremic patients, of whom 28 were receiving antibiotics at the time their cultures were performed. Overall, ARD processing neither increased the isolation rate nor decreased the time interval required for detection of organisms compared with conventional processing. ARD processing was the only means of isolation of organisms from only seven of the 87 patients and three of the 28 receiving antibiotics, whereas conventional processing accounted for the only means of isolation of organisms from 17 of the 87 patients and nine of the 28 receiving antibiotics. ARD processing demonstrated no advantages over conventional processing of blood cultures.