A regimen of a single intramuscular dose of penicillin G-streptomycin was compared with regimens of three oral doses of amoxicillin and two oral doses of penicillin V to prevent Streptococcus sanguis endocarditis in rabbits with experimentally induced valvular heart lesions. Challenge doses of 10(4), 10(6), and 10(8) CFU of a strain of S. sanguis highly tolerant to penicillin and amoxicillin were used. The combination of penicillin and streptomycin was the only regimen tested that provided full protection even against the highest inoculum concentration. A single oral dose of penicillin V (36 mg/kg) or amoxicillin (50 mg/kg), two oral doses of penicillin V (36 and 18 mg/kg with a 7-h interval between doses), or six oral doses of amoxicillin (50 mg/kg followed by 8.5 mg/kg at 8-h intervals) protected recipients of the lowest inoculum concentration; protection diminished with increasing inocula. In contrast, administration of two high oral doses of amoxicillin (50 mg/kg) with a 10-h interval between doses provided full protection against challenge doses of 10(4) and 10(6) CFU, preventing endocarditis in 10 (66%) of 15 recipients of 10(8) CFU. All regimens evaluated were highly effective in preventing endocarditis when rabbits were challenged with 10(4) CFU. The combination of penicillin and streptomycin was the best regimen tested. Administration of two high oral doses of amoxicillin (50 mg/kg) with a 10-h interval between doses led to significantly fewer infections when compared with the other oral regimens when rabbits were challenged with 10(6) and 10(8) CFU.
This study compares the effects of a total daily dose of gentamicin given once a day (q.d.) or three times a day (t.i.d.) in the therapy of experimental endocarditis in rabbits caused by penicillin-susceptible, penicillin-tolerant, or penicillin-resistant viridans streptococci. Four isolates were used in vivo: one penicillin susceptible (MIC < or = 0.03 microgram/ml), one penicillin tolerant (MBC/MIC, < or = 0.03/ > 32 micrograms/ml), and two penicillin resistant (MICs = 0.5 and 2 micrograms/ml). Animals were infected with one of the four isolates and assigned to one of the following treatment regimens: no treatment, procaine penicillin at 1.2 million IU intramuscularly (i.m.) t.i.d., procaine penicillin plus gentamicin at 1 mg/kg of body weight i.m. t.i.d., procaine penicillin plus gentamicin at 3 mg/kg i.m. q.d., or procaine penicillin plus gentamicin at 1 mg/kg i.m. q.d. (only animals infected with the penicillin-susceptible isolate). Serum drug concentrations measured 30 min after administration of 1.2 million IU of penicillin and 1 or 3 mg of gentamicin per kg were 22.6, 3.8, and 8.5 micrograms/ml, respectively. The reduced total daily dose of gentamicin was ineffective among animals infected with penicillin-susceptible viridans streptococci; treatment with 1 mg of gentamicin per kg per day plus penicillin was less effective (P < 0.05) than was treatment with 3 mg of gentamicin per kg per day plus penicillin. The 1-mg/kg/day gentamicin treatment regimen was not further studied. The gentamicin dosing interval did not significantly affect (q.d. versus t.i.d., P > 0.05) the relative efficacy of penicillin plus gentamicin for treatment of experimental endocarditis among animals infected with each of the four isolates tested.
The effectiveness of various antibiotics commonly recommended for the prophylaxis of bacterial endocarditis has been evaluated in experimental streptococcal endocarditis in rabbits. High doses of penicillin G did not prevent the development of this infection. The only consistently successful prophylactic regimens using penicillin alone were those which provided for both an early high serum level and more than 9 h of effective antimicrobial action. Vancomycin was the only other drug which proved uniformly successful when given alone, even though the duration of its antimicrobial action in the blood was only 3 h. However, combined therapy using penicillin G or ampicillin with streptomycin was always effective in prophylaxis. Treatment with single injections of ampicillin, cephaloridine, cephalexin, clindamycin, cotrimoxazole, rifampicin, streptomycin, erythromycin, and tetracycline failed to prevent infection.
The findings provide information on the effect of antimicrobials in vivo and may be applicable to the chemoprophylaxis of infective endocarditis in clinical practice.
The pharmacokinetics and bacteriological efficacies of penicillin G, ceftriaxone, vancomycin, and imipenem were determined in rabbits with experimental meningitis caused by Streptococcus pneumoniae strains with different penicillin susceptibilities. Drug dosages were adjusted to attain peak concentrations in serum that were similar to those observed in infants and children. In animals infected with a penicillin-susceptible (MBC, 0.008 micrograms/ml) pneumococcus, penicillin G and ceftriaxone reduced the number of organisms in cerebrospinal fluid (CSF) by greater than or equal to 4.14 log10 CFU/ml after single doses and after 9-h continuous infusions. A single large dose (50 mg/kg) of penicillin G was comparatively ineffective (-2.15 log10 CFU/ml) against a relatively penicillin-resistant (MBC, 0.5 micrograms/ml) strain, whereas ceftriaxone therapy resulted in a 3.66- and 4.77-log10 CFU/ml reduction after single doses and 9-h continuous infusions, respectively. In animals in which meningitis was caused by a penicillin-resistant (MBC, 8.0 micrograms/ml) pneumococcus, a single dose of penicillin (50 or 150 mg/kg) or of ceftriaxone failed to lower the number of organisms in CSF. Vancomycin and imipenem reduced the counts in CSF by at least 2.19 and 4.10 log10 CFU/ml after single doses and 9-h infusions, respectively. In all experiments, a bactericidal titer of greater than or equal to 1:8 in CSF was necessary to achieve a maximal bacteriological effect.
Previous in vitro studies demonstrating that the penicillinase-resistant penicillins act synergistically in combination with gentamicin against some enterococci have suggested that these combinations might be effective therapy for enterococcal infections in vivo. To determine the in vivo effectiveness of such combinations, we treated rabbits with enterococcal endocarditis with gentamicin and either nafcillin, oxacillin, or methicillin. Despite doses of the penicillins equivalent to 12 or 24 g/day in a 70-kg patient, the percentage of animals in each treatment group with sterile valves at autopsy after spontaneous death or sacrifice after 21 days of therapy was low. High-dose therapy with the penicillins did not significantly increase survival over the low-dose treatment groups. Thus, it seems prudent to include penicillin with a penicillinase-resistant penicillin and gentamicin as the initial therapy of patients with endocarditis possibly caused by enterococci.
The treatment of experimental gas gangrene caused by Clostridium perfringens was investigated by using combinations of antimicrobial agents. This study demonstrated that rifampin, penicillin, metronidazole, and clindamycin were all bactericidal against standard inocula (10(5) to 10(6) CFU). These antimicrobial agents were then administered to mice beginning 30 min after intramuscular injection of 10(9) CFU of C. perfringens type A. The highest doses used produced levels of drug in blood which exceeded the MIC by at least a factor of 40. In addition, other groups of mice received monotherapy at full dose or one-fourth full dose or combination antimicrobial therapy at full or one-fourth full dose. Among the single and combination antimicrobial treatments, metronidazole alone, clindamycin alone, and clindamycin plus penicillin were the most efficacious (P less than 0.05). Although the survival of mice treated with clindamycin plus penicillin was greater than that of mice treated with clindamycin alone, the difference did not reach statistical significance (P greater than 0.05). In contrast, mice treated with a combination of metronidazole and penicillin demonstrated greater mortality than those treated with metronidazole alone (P less than 0.05). In summary, combination antimicrobial therapy of experimental C. perfringens infection did not improve survival compared to that achieved with metronidazole or clindamycin alone, and some combinations significantly reduced survival (P less than 0.05).
The effectiveness of cephalexin, an oral cephalosporin using a dosage equivalent to available capsular dosage forms, was studied in relation to the effectiveness of phenoxymethyl penicillin and benzathine penicillin in the treatment of 128 patients with beta-hemolytic streptococcal pharyngitis, all but six of whom had group A streptococci isolated from throat cultures. Approximately one-half, 66 patients, received cephalexin for 10 days; 34 patients received phenoxymethyl penicillin for 10 days; and 28 patients had a single injection of benzathine penicillin. There were four treatment failures determined bacteriologically post-therapy, two in the cephalexin treatment group and one each in the oral penicillin and intramuscular penicillin groups. Similar cure rates of 96.7, 97.1, and 96.4% were computed for the respective treatment regimens. Whereas intramuscular benzathine penicillin remains the regimen of choice in most instances, cephalexin appeared to be as effective as oral penicillin in the elimination of group A streptococci from the pharynx when oral treatment was desired for streptococcal pharyngitis.
Pneumococcal pneumonia in rats with intact host defense mechanisms could be successfully cured by penicillin. The efficacy of this antibiotic therapy was lost in cobra venom factor-treated rats which had selectively impaired phagocytic functions. In these animals the effect of penicillin therapy was improved by increasing the daily dose and the frequency of injections and by earlier initiation of the therapy. The efficacy of penicillin in the cobra venom facttor-treated rats was restored either by markedly increasing the daily dose of penicillin or by increasing the daily dose in combination with a reduced interval of the penicillin injections.
We used a mouse model of pneumococcal pneumonia to assess the bactericidal effect of increasing doses of amoxicillin (AMX) against clinical strains with various susceptibilities to penicillin. Twelve strains that exhibited similar virulence in mice were selected. Three were penicillin susceptible (PS) (penicillin and AMX MICs = 0.01 to 0.03 microgram/ml), three were intermediately resistant (PIR) (penicillin and AMX MICs = 0.5 to 1 microgram/ml), and six were penicillin resistant (PR) (penicillin and AMX MICs = 1 to 8 micrograms/ml). Leukopenic Swiss mice were infected intratracheally with 10(7) CFU of each strain. Treatment was initiated 3 h after infection and consisted of a single subcutaneous injection of AMX at doses ranging from 2.5 to 10 mg/kg (PS strains), 5 to 100 (PIR strains), and 25 to 3,000 (PR strains). Bacterial killing kinetics were recorded in the lungs over 9 h. The maximal log CFU reduction (Emax) was observed 3 h postinjection. The relation between Emax and log10(dose/MIC) showed two populations. With seven strains (the three PS, the three PIR, and one of the six PR [MICs, penicillin/AMX = 4/1]) a good correlation was observed between Emax and log10(dose/MIC) (r = 0.772; P < 0.02). A bactericidal effect equal to 3.5 log10 CFU was observed at a log10(dose/MIC) = 2. At this ratio, with the five other PR strains, Emax varied from 0.4 to 1.6 log10 CFU. In brain heart infusion medium containing AMX at 50 times the relevant MIC, these five PR strains were tolerant in vitro. Treatment failure with AMX was found in vivo, with tolerant, highly resistant strains.
Single-dose antimicrobial therapy has clear advantages over multiple-dose therapy. Long-acting penicillins have been used for many years in single doses for treatment of streptococcal pharyngitis and early syphilis. More recently, shorter-acting agents are used for non-invasive mucosal infections. In trichomonas vaginitis, for instance, a 2g single dose of metronidazole is approximately 92% effective and is considered the treatment of choice. Controversy still exists about the value of single-dose therapy in women who have bacterial cystitis. However, there is good evidence that 2 or 3 double-strength tablets of co-trimoxazole are very effective and safe in the treatment of uncomplicated cystitis in healthy women.
infectious diseases; single-dose antimicrobial therapy; cystitis
The ability of antibiotics to prevent Streptococcus sanguis endocarditis was tested in rabbits. Only vancomycin or a combination of penicillin G plus streptomycin always prevented infection when administered as a single dose. A loading dose of 30 mg/kg of phenoxymethyl penicillin (penicillin V) followed by additional 7.5 mg/kg doses for 48 h proved to be the only successful prophylactic program that could be given orally to man. Cefazolin alone or with streptomycin in multiple doses was also an effective alternative to penicillin or penicillin derivatives. Erythromycin uniformly failed to protect animals from bacterial endocarditis but showed greater prophylactic efficacy when a low inoculum of streptococci was used.
An investigation was undertaken to discover whether a single intramuscular dose of long-acting (or mixed long-acting and crystalline) penicillin or a single day's therapy with oral penicillin was satisfactory treatment for lobar pneumonia. These treatments were compared with standard hospital oral and injection therapies. All the experimental treatment regimes were found to be satisfactory. They provide justification for treating lobar pneumonia on an out-patient basis in order to save hospital admissions.
Perioperative antimicrobial administration practices were evaluated retrospectively in 97 horses undergoing elective arthroscopy, and antimicrobial use was compared with standard recommendations for perioperative prophylaxis. Parenteral antimicrobials were administered perioperatively to 95/97 (98%) horses, 88 of which received intravenous sodium penicillin. Time from 1st dose until 1st incision ranged from 30 to 390 min [142 ± 55.6 min, mean ± standard deviation (s), median 135 min], and the first incision was performed greater than 2 half-lives after administration of sodium penicillin in 86/95 (91%) cases. Overall duration of therapy was 30.8 ± 24.2 h (mean ± s). Six (6.3%) horses received only a single preoperative dose, while 63 (66%) horses were treated for 24 h or less. While objective data regarding optimal perioperative antimicrobial prophylaxis are limited, the antimicrobial use practices observed here commonly deviated from standard recommendations for perioperative prophylaxis.
Group B beta-hemolytic streptococci (GB-BHS) frequently cause severe infection in newborns. Previous in vitro studies showed accelerated killing of GB-BHS by ampicillin plus gentamicin as compared with ampicillin alone. To extend the in vitro observations, mice were infected experimentally with GB-BHS and treated with gentamicin plus ampicillin or ampicillin alone. Untreated mice died within 10 to 48 h. Compared with treatment with ampicillin alone, ampicillin-and-gentamicin therapy resulted in improved survival when the antibiotics were given in established infection or as a single dose at the time of infection. Ampicillin and gentamicin accelerated the clearing of bacteremia as compared with treatment with ampicillin alone. In view of these findings, the therapy of GB-BHS infection in newborns and other patients should be reconsidered in that a combination of ampicillin or penicillin G plus gentamicin might be superior to the use of ampicillin or penicillin G alone.
Gonorrhea remains an important clinical and public health problem throughout the world. Gonococcal infections have historically been diagnosed by Gram stain and culture, but are increasingly diagnosed through nucleic acid tests thereby eliminating the opportunity for antimicrobial susceptibility testing. Gonococcal infections are typically treated with single-dose therapy with an agent found to cure >95% of cases. Unfortunately, the gonococcus has repeatedly developed resistance to antimicrobials including sulfonamides, penicillin, tetracyclines, and fluoroquinolones. This has left third-generation cephalosporins as the lone class of antimicrobials currently recommended as first line therapy for gonorrhea in some regions. However, resistance to oral third-generation cephalosporins has emerged and spread in Asia, Australia and elsewhere. The mechanism of this resistance seems to be associated with a mosaic penicillin binding protein (penA) in addition to other chromosomal mutations previously found to confer resistance to beta-lactam antimicrobials (ponA, mtrR, penB, pilQ). Few good options exist or are in development for treating cephalosporin resistant isolates as most have had multidrug resistance. Preventing the spread of resistant isolates will depend on ambitious antimicrobial management programs, strengthening and expanding surveillance networks, and through effective sexually transmitted disease control and prevention.
Neisseria gonorrhoeae; cephalosporin resistance
126 female patients with proven gonococcal infection were treated with 4 g. spectinomycin bihydrochloride given intramuscularly in a single does. In 120 cases followed up there were four possible treatment failures, giving a cure rate of 97-6 per cent. 99 male patients with acute gonococcal urethritis were treated with a single dose of 2, g. sectinomycin bihydrochloride. Of 88 cases followed up, there were three possible treatment failures, giving a cure rate of 96-6 per cent. Spectinomycin is an important addition to gonorrhoea therapy but should be reserved for penicillin sensitive patients and penicillin insensitive infections.
The efficacy of ceftriaxone or penicillin alone or combined with gentamicin at different dosing intervals was evaluated in experimental endocarditis due to a penicillin-susceptible, ceftriaxone-tolerant strain of Streptococcus sanguis I. The difference between monotherapy with ceftriaxone and procaine penicillin approached statistical significance (P = 0.052). Ceftriaxone combined with gentamicin administered as a single daily dose was less effective than was procaine penicillin combined with gentamicin administered in a single daily dose or in three divided doses.
The penicillin-aminoglycoside combination is recommended for the treatment of systemic enterococcal infections. However, the optimal dosing regimen of the aminoglycoside remains to be elucidated. We evaluated the efficacy of penicillin, alone or in combination with various dosing regimens of netilmicin, for the treatment of experimental left-sided Enterococcus faecalis endocarditis in rabbits. Animals were injected intramuscularly for 4 days with penicillin alone or in combination with netilmicin in one of the following regimens: netilmicin at a low dose (2 mg/kg of body weight every 8 h), netilmicin at a high dose (4 mg/kg every 8 h), or netilmicin at a single daily high dose (12 mg/kg every 24 h). MICs and MBCs were 3.1 and 6.2 micrograms/ml and 8 and 8 micrograms/ml for penicillin and netilmicin, respectively. A netilmicin concentration of 4 micrograms/ml was the lowest concentration that achieved synergism with penicillin, as shown by the kill-curve method. Mean peak levels of netilmicin in serum were 5.6 (netilmicin at 2 mg/kg), 9.8 (netilmicin at 4 mg/kg), and 20.6 (netilmicin at 12 mg/kg) micrograms/ml. Mean penicillin levels in serum were constantly above the MIC. Penicillin plus netilmicin at a high dose given three times daily was more effective (P less than 0.05) than any other regimen in reducing bacterial titers in vegetations and was the only treatment that induced a significant bactericidal activity in rabbit serum during the trough. We concluded that divided doses of aminoglycoside are more effective than the same total dose given once daily in combination with penicillin. Our data suggest that prolonged levels of aminoglycoside in serum might be important to exhibit the greatest in vivo efficacy of the combination against E. faecalis. They also indicate that use of a reduced total daily dose of aminoglycoside or an increase in the interval between each dose might reduce the efficacy of therapy in animals with this type of infection.
The combination of penicillin and aminoglycoside is the recommended therapy for endocarditis caused by nutritionally variant streptococci (NVS). However, the optimal aminoglycoside dosing regimen remains controversial. We compared the efficacies of four regimens of tobramycin alone or combined with procaine penicillin in the therapy of rabbits with endocarditis caused by Streptococcus adjacens, a new species of NVS. Animals were injected intramuscularly for 4 days with procaine penicillin (150,000 U/kg of body weight twice daily) or tobramycin at a low dose (3 mg/kg every 24 h) or a high dose (12 mg/kg every 24 h) either once or three times daily (t.i.d.) alone or in combination with procaine penicillin. Additional groups of animals were treated with the combination regimens for a shorter period of time (2 days) in order to demonstrate a possible difference in the rapidity of efficacy between the regimens. The MICs and MBCs were 0.015 and 1 micrograms/ml and 8 and 16 micrograms/ml for penicillin and tobramycin, respectively. The mean peak tobramycin levels in plasma were 2.4 +/- 1.3 (1 mg/kg t.i.d.), 5.4 +/- 3.7 (4 mg/kg t.i.d.), and 25 +/- 9.3 (12 mg/kg once daily). The mean penicillin levels in serum were always above the MIC. In vitro kill curves plotted at the time that peak concentrations were reached in plasma showed a concentration-dependent killing effect of tobramycin alone but not in combination with penicillin. In vivo, low-dose tobramycin was significantly less effective than the high dose. Results for the combinations of the different dosing regimens of tobramycin with procaine penicillin were not significantly different. Our results suggest that (i) against susceptible strains of streptococci, aminoglycoside alone exhibits a concentration-dependent killing effect both in vitro and in vivo; (ii) against NVS strains, combinations of penicillin and high- or low-dose tobramycin are equally effective; and (iii) aminoglycoside given once daily or at a low dose t.i.d. with penicillin could be a cost-effective alternative with reduced toxic risk for patients with NVS endocarditis when the bacteria are susceptible to the killing activities of both compounds.
The bactericidal activities of penicillin G and ampicillin alone were compared with those of their combinations with streptomycin or gentamicin against 17 strains of lactobacilli classified as tolerant to various beta-lactam antibiotics. The penicillin G combinations with streptomycin and gentamicin were synergistic against 17 and 16 of these strains, respectively, whereas the corresponding ampicillin-aminoglycoside combinations were synergistic against 12 and 15 strains, respectively. Importantly, synergy was manifested at concentrations of these antibiotics that are attained in serum after their administration in conventional dose regimens. In no instances were combinations antagonistic. These in vitro observations provide a partial explanation for the favorable results obtained in preliminary clinical evaluations of the benefits of combination regimens in the treatment of lactobacillus infections refractory to single-drug therapy.
Four oral penicillin V regimens were compared for the ability to prevent Streptococcus sanguis infection of experimentally induced valvular heart lesions in rabbits. Challenge doses of 10(4), 10(6), and 10(8) CFU of a penicillin-susceptible strain of S. sanguis were used in this study. Measured by recovery of test organisms from endocardial lesions, the lowest-concentration inoculum was infective for 53% of the recipients; the higher-concentration inocula were infective for all recipients. A single-oral-dose penicillin V regimen (36 mg/kg of body weight) prevented endocarditis when rabbits were challenged with 10(4) CFU, but protection diminished with increasing inoculum concentrations. In contrast, addition of a second penicillin V dose (18 mg/kg of body weight) administered with a 7-h interval between doses achieved fully effective prophylaxis against even the highest inoculum tested (10(8) CFU). A repeated set of experiments in which half the dose of penicillin V was administered showed significantly reduced protection against S. sanguis endocarditis.
Spectinomycin hydrochloride, a new parenteral antibiotic prepared from Streptomyces spectabilis, was compared with standard U.S. Public Health Service-recommended dosages of aqueous procaine penicillin G in the treatment of uncomplicated gonorrhea in 353 men and 314 women. Of the 314 women, 130 had a pretreatment positive rectal culture. All diagnoses were proven by culture on Thayer-Martin selective medium. Minimal inhibitory concentrations of both drugs were determined. Single doses of 2 and 4 g of spectinomycin were compared with 2.4 million units of procaine penicillin in males and with both 2.4 and 4.8 million units of procaine penicillin in females. Both spectinomycin schedules, 2.4 million units of penicillin in males and 4.8 million units of penicillin in females, resulted in cure rates in excess of 90%. There were no failures at the rectal site only in those women with positive rectal cultures. There was no advantage to using the larger amount of spectinomycin in either sex.
Six hundred and thirteen male patients with acute uncomplicated gonorrhoea have been treated alternately with single injections of 1·2 mega-units of aqueous procaine penicillin alone or with the same dose and preparation of penicillin with an additional 1·0 g of probenecid given orally immediately prior to injection.
Whether the failure rates were assessed on the basis of an absence of further sexual exposure or by classifying all recurrences within 1 or 2 weeks regardless of history as treatment failures the results were significantly better when probenecid was also given.
The use of a single dose of probenecid is thus capable of maintaining the success of single injection procedures for the treatment of gonorrhoea based on procaine penicillin and thus represents a bulwark for the future should the powers of penicillin against the gonococcus in London deteriorate further.
Serum and subcutaneous chamber fluid (CF) dynamics of penicillin G, ampicillin, and amoxicillin were studied in rabbits after single large parenteral doses comparable to doses used in treating gonorrhea and endocarditis. The effects of parenteral probenecid and of injection of an antibiotic directly into a subcutaneous chamber (“intrachamber” injection) also were studied. Peak serum antibiotic concentrations exceeded peak CF concentrations and occurred sooner. Antimicrobial activity persisted longer in CF than in serum. Percent penetration [100 × (CF peak/serum peak)] of CF was least after intramuscular ampicillin and amoxicillin, was greatest after intrachamber ampicillin and intrąmuscular aqueous procaine penicillin G, and was related to duration of antibiotic concentration gradients from serum to CF. Intramuscular aqueous crystalline penicillin G resulted in higher serum and CF penicillin G concentrations than intramuscular aqueous procaine penicillin G, which prolonged the duration of penicillin G in serum and CF. Amoxicillin diffused into CF more readily than ampicillin. Probenecid resulted in higher early serum and CF antibiotic concentrations, but had little or no effect on duration of antibiotic activity. Intrachamber ampicillin resulted in more prolonged serum and CF ampicillin activity than intramuscular ampicillin, but much lower peak serum concentrations. The data suggest a possible means by which probenecid improves the efficacy of gonorrhea therapy with aqueous procaine penicillin G. Intrachamber administration of penicillins could be useful in treating experimental infections requiring prolonged therapy.
The effectiveness of ziracin (SCH27899), a novel everninomicin, was at first investigated against lethal pneumonia caused by a penicillin-susceptible Streptococcus pneumoniae strain. A single intravenous injection of ziracin at a dose of 60 mg/kg of body weight given at 18 h postinfection protected 100% mice and led to the complete clearance of bacteria from their lungs. The activity of ziracin was observed to be the same as that of ceftriaxone: the 50% protective doses (PD50s) of ziracin and ceftriaxone were 24.8 and 24.6 mg/kg, respectively. Evaluation of this therapy with leukopenic mice showed that a single injection of ziracin protected 75% of these mice. A delay in therapy with ziracin, which was initiated at 48 h postinfection with 30 mg/kg given once daily for 3 days, resulted in an 83% survival rate of immunocompetent mice. The efficacy of ziracin was further compared to that of vancomycin against lethal pneumonia caused by a penicillin-resistant S. pneumoniae strain in leukopenic mice. The PD50s of ziracin and vancomycin were 40.5 and 44.2 mg/kg, respectively. Treatment with ziracin at 30 mg/kg once daily for 2 days (initiated 18 h postinfection) yielded an 83% survival rate and achieved complete eradication of the bacteria. The results were the same as those obtained with vancomycin administered at 15 mg/kg twice daily for 2 days. It is notable that the high survival rates for mice treated with ziracin were associated with effective eradication of the bacteria and rapid recovery of pulmonary tissues from pneumonia. The pharmacokinetic properties of ziracin, ceftriaxone, and vancomycin were estimated following intravenous administration of a single dose of 30 mg/kg to immunocompetent mice. The half-life of ziracin was observed to be longer than those of ceftriaxone and vancomycin (2.3 h versus 1.0 and 0.36 h in the bloodstream and 3 h versus 1.9 and 0.45 h in lung tissues). The areas under the concentration-time curves (AUCs) in lung tissue for ziracin versus those for ceftriaxone and vancomycin were 36 μg · h/g versus 20 and 9.5 μg · h/g. The prolonged half-life and high AUC for ziracin in tissue contributed to its excellent in vivo activities.