Influence of subinhibitory concentrations of penicillin, cephalothin, and clindamycin on Staphylococcus aureus growth in human phagocytic cells

Staphylococcal mastitis in dairy cows

Bovine mastitis is one of the most common diseases of dairy cattle. Even though different infectious microorganisms and mechanical injury can cause mastitis, bacteria are the most common cause of mastitis in dairy cows. Staphylococci, streptococci, and coliforms are the most frequently diagnosed etiological agents of mastitis in dairy cows. Staphylococci that cause mastitis are broadly divided into Staphylococcus aureus and non-aureus staphylococci (NAS). NAS is mainly comprised of coagulase-negative Staphylococcus species (CNS) and some coagulase-positive and coagulase-variable staphylococci. Current staphylococcal mastitis control measures are ineffective, and dependence on antimicrobial drugs is not sustainable because of the low cure rate with antimicrobial treatment and the development of resistance. Non-antimicrobial effective and sustainable control tools are critically needed. This review describes the current status of S. aureus and NAS mastitis in dairy cows and flags areas of knowledge gaps.

Sub-Inhibitory Concentrations of Oxacillin, but Not Clindamycin, Linezolid, or Tigecycline, Decrease Staphylococcal Phenol-Soluble Modulin Expression in Community-Acquired Methicillin-Resistant Staphylococcus aureus

Staphylococcus aureus (SA) is a major human pathogen producing virulence factors, such as Panton-Valentine-leucocidin (PVL), alpha-hemolysin (Hla), and phenol-soluble-modulins alpha (PSMα), including delta-hemolysin (Hld). Unlike oxacillin, clindamycin and linezolid subinhibitory concentrations (sub-MIC) display an anti-toxin effect on PVL and Hla expression. Few studies have investigated PSMα and Hld expression modulation by antibiotics. Herein, we assessed the effect of antibiotic sub-MIC on PSMα1 and Hld expression for 4 community-acquired methicillin-resistant SA (CA-MRSA), 2 strains belonging to USASA300 and 2 strains belonging to ST80 European clone. SA were grown under oxacillin, clindamycin, linezolid, or tigecycline. After incubation, culture pellets were used for the determination of psmα1, pmtB, pmtR mRNA, and RNAIII levels by relative quantitative RT-PCR. PSMα1 and Hld expressions were measured in supernatant using high-performance-liquid-chromatography coupled to mass-spectrometry (HPLC-MS). Oxacillin sub-MIC reduced PSMα1 and Hld production, partially related to mRNA variations. For other antibiotics, effects on toxin expression were strain or clone dependent. Antibiotic effect on mRNA did not always reflect protein expression modulation. Variations of pmtB, pmtR mRNA, and RNAIII levels were insufficient to explain toxin expression modulation. Altogether, these data indicate that PSMα and Hld expressions are modulated by antibiotics (potential anti-toxin effect of oxacillin) differently compared to PVL and Hla.

IMPORTANCE Staphylococcal toxins play an important role in the physiopathology of staphylococcal infections. Subinhibitory concentrations (sub-MIC) of antibiotics modulate in vitro toxins expression in S. aureus: clindamycin (CLI) and linezolid (LIN) display an anti-toxin effect on Panton-Valentine leucocidin and alpha-hemolysin production, while oxacillin (OXA) has an inducing effect. Few studies have focused on the modulation of phenol-soluble modulins alpha (PSMα) including delta-hemolysin expression by sub-MIC antibiotics. The aim of the present study was to investigate the effects of sub-MIC antibiotics on the expression of PSMα toxins for 4 community-acquired methicillin-resistant S. aureus (CA-MRSA) clinical isolates. The data presented herein confirm that OXA sub-MICs constantly inhibit PSMα production for CA-MRSA. Certain strains of S. aureus are highly sensitive to sub-MICs of protein synthesis inhibitory agents, resulting in an important increase of mRNA levels to overcome the intrinsic ribosome blockage ability of these antibiotics, eventually translating in increased expression of toxins.

Pharmacodynamic Evaluation of a Single Dose versus a 24-Hour Course of Multiple Doses of Cefazolin for Surgical Prophylaxis

The optimal perioperative duration for the administration of cefazolin and other prophylactic antibiotics remains unclear. This study aimed to describe the pharmacodynamics of cefazolin for a single 2 g dose versus a 24 h course of a 2 g single dose plus a 1 g eight-hourly regimen against methicillin-susceptible Staphylococcus aureus. Static concentration time-kill assay and a dynamic in vitro hollow-fibre infection model simulating humanised plasma and interstitial fluid exposures of cefazolin were used to characterise the pharmacodynamics of prophylactic cefazolin regimens against methicillin-sensitive Staphylococcus aureus clinical isolates. The initial inoculum was 1 × 105 CFU/mL to mimic a high skin flora inoculum. The static time-kill study showed that increasing the cefazolin concentration above 1 mg/L (the MIC) did not increase the rate or the extent of bacterial killing. In the dynamic hollow-fibre model, both dosing regimens achieved similar bacterial killing (~3-log CFU/mL within 24 h). A single 2 g dose may be adequate when low bacterial burdens (~104 CFU/mL) are anticipated in an immunocompetent patient with normal pharmacokinetics.

NOX2 Deficiency Permits Sustained Survival of S. aureus in Macrophages and Contributes to Severity of Infection

Although the crucial role of professional phagocytes for the clearance of S. aureus infections is well-established, several studies indicate an adverse role of leukocytes in the dissemination of S. aureus during infection. Since only little is known about macrophages in this context, we analyzed the role of macrophages, and in particular reactive oxygen species deficiency, for the seeding of S. aureus metastases. Infection of bone marrow-derived macrophages (BMDM) with S. aureus revealed that NADPH oxidase 2 (NOX2-) deficient, but not NOX1- or NOX4-deficient, BMDM failed to clear intracellular S. aureus. Despite of larger intracellular bacterial burden, NOX2-deficient BMDM showed significantly improved survival. Intravenous injection of mice with in vitro-infected BMDMs carrying intracellular viable S. aureus led to higher bacterial loads in kidney and liver of mice compared to injection with plain S. aureus. An even higher frequency of liver abscesses was observed in mice infected with S. aureus-loaded nox2 ^-/- BMDM. Thus, the improved intracellular survival of S. aureus and improved viability of NOX2-deficient BMDM is associated with an aggravated metastatic dissemination of S. aureus infection. A combination of vancomycin and the intracellularly active antibiotic rifampicin led to complete elimination of S. aureus from liver within 48 h, which was not achieved with vancomycin treatment alone, underscoring the impact of intracellular S. aureus on the course of disease. The results of our study indicate that intracellular S. aureus carried by macrophages are sufficient to establish a systemic infection. This suggests the inclusion of intracellularly active antibiotics in the therapeutic regimen of invasive S. aureus infections, especially in patients with NADPH oxidase deficiencies such as chronic granulomatous disease.

Antimicrobial Treatment of Peritonitis Associated with Continuous Ambulatory Peritoneal Dialysis

A multitude of therapeutic regimens have been proposed for the management of peritonitis associated with continuous ambulatory peritoneal dialysis (CAPD). There are, however, few clinical trials that have evaluated the efficacy of these proposed regimens in a prospective, comparative fashion. This retrospective report is a tabulation of the published data on antimicrobial treatment of CAPD-related peritonitis. The results are presented for combination and mono-drug therapies; Gram-positive bacterial, Gram -negative bacterial and fungal infections; intravenous, oral and intraperitoneal (i.p.) routes of drug administration; various dosages and dosing intervals; and clinical response and relapse rates. The apparent optimal combination regimen for empiric treatment of peritonitis is vancomycin administered in 1 dialysis exchange/week with ceftazidime. This regimen avoids the toxicity associated with the use of aminoglycosides while maintaining effectiveness.

Staphylococcus aureus facilitates its survival in bovine macrophages by blocking autophagic flux

Staphylococcus aureus is a pathogen that is the causative agent of several human and veterinary infections and plays a critical role in the clinical and subclinical mastitis of cattle. Autophagy is a conserved pathogen defence mechanism in eukaryotes. Studies have reported that S aureus can subvert autophagy and survive in cells. Staphylococcus aureus survival in cells is an important cause of chronic persistent mastitis infection. However, it is unclear whether S aureus can escape autophagy in innate immune cells. In this study, initiation of autophagy due to the presence of S aureus was detected in bovine macrophages. We observed autophagic vacuoles increased after S aureus infection of bovine macrophages by transmission electron microscopy (TEM). It was also found that S aureus‐infected bovine macrophages increased the expression of LC3 at different times(0, 0.5, 1, 1.5, 2, 2.5, 3 and 4 hours). Data also showed the accumulation of p62 induced by S aureus infection. Application of autophagy regulatory agents showed that the degradation of p62 was blocked in S aureus induced bovine macrophages. In addition, we also found that the accumulation of autophagosomes promotes S aureus to survive in macrophage cells. In conclusion, this study indicates that autophagy occurs in S aureus‐infected bovine macrophages but is blocked at a later stage of autophagy. The accumulation of autophagosomes facilitates the survival of S aureus in bovine macrophages. These findings provide new insights into the interaction of S aureus with autophagy in bovine macrophages.

Intracellular Penetration and Effects of Antibiotics on Staphylococcus aureus Inside Human Neutrophils: A Comprehensive Review

Neutrophils are important assets in defense against invading bacteria like staphylococci. However, (dysfunctioning) neutrophils can also serve as reservoir for pathogens that are able to survive inside the cellular environment. Staphylococcus aureus is a notorious facultative intracellular pathogen. Most vulnerable for neutrophil dysfunction and intracellular infection are immune-deficient patients or, as has recently been described, severely injured patients. These dysfunctional neutrophils can become hide-out spots or “Trojan horses” for S. aureus. This location offers protection to bacteria from most antibiotics and allows transportation of bacteria throughout the body inside moving neutrophils. When neutrophils die, these bacteria are released at different locations. In this review, we therefore focus on the capacity of several groups of antibiotics to enter human neutrophils, kill intracellular S. aureus and affect neutrophil function. We provide an overview of intracellular capacity of available antibiotics to aid in clinical decision making. In conclusion, quinolones, rifamycins and sulfamethoxazole-trimethoprim seem very effective against intracellular S. aureus in human neutrophils. Oxazolidinones, macrolides and lincosamides also exert intracellular antibiotic activity. Despite that the reviewed data are predominantly of in vitro origin, these findings should be taken into account when intracellular infection is suspected, as can be the case in severely injured patients.

The Role of Antibiotics in Modulating Virulence in Staphylococcus aureus

Staphylococcus aureus is often involved in severe infections, in which the effects of bacterial virulence factors have great importance. Antistaphylococcal regimens should take into account the different effects of antibacterial agents on the expression of virulence factors and on the host's immune response. A PubMed literature search was performed to select relevant articles on the effects of antibiotics on staphylococcal toxin production and on the host immune response. Information was sorted according to the methods used for data acquisition (bacterial strains, growth models, and antibiotic concentrations) and the assays used for readout generation. The reported mechanisms underlying S. aureus virulence modulation by antibiotics were reviewed. The relevance of in vitro observations is discussed in relation to animal model data and to clinical evidence extracted from case reports and recommendations on the management of toxin-related staphylococcal diseases. Most in vitro data point to a decreased level of virulence expression upon treatment with ribosomally active antibiotics (linezolid and clindamycin), while cell wall-active antibiotics (beta-lactams) mainly increase exotoxin production. In vivo studies confirmed the suppressive effect of clindamycin and linezolid on virulence expression, supporting their utilization as a valuable management strategy to improve patient outcomes in cases of toxin-associated staphylococcal disease.

Fosfomycin enhances phagocyte-mediated killing of Staphylococcus aureus by extracellular traps and reactive oxygen species

The successful treatment of bacterial infections is the achievement of a synergy between the host's immune defences and antibiotics. Here, we examined whether fosfomycin (FOM) could improve the bactericidal effect of phagocytes, and investigated the potential mechanisms. FOM enhanced the phagocytosis and extra- or intracellular killing of S. aureus by phagocytes. And FOM enhanced the extracellular killing of S. aureus in macrophage (MФ) and in neutrophils mediated by extracellular traps (ETs). ET production was related to NADPH oxidase-dependent reactive oxygen species (ROS). Additionally, FOM increased the intracellular killing of S. aureus in phagocytes, which was mediated by ROS through the oxidative burst process. Our results also showed that FOM alone induced S. aureus producing hydroxyl radicals in order to kill the bacterial cells in vitro. In a mouse peritonitis model, FOM treatment increased the bactericidal extra- and intracellular activity in vivo, and FOM strengthened ROS and ET production from peritoneal lavage fluid ex vivo. An IVIS imaging system assay further verified the observed in vivo bactericidal effect of the FOM treatment. This work may provide a deeper understanding of the role of the host's immune defences and antibiotic interactions in microbial infections.

Printed paper-based arrays as substrates for biofilm formation

The suitability of paper-based arrays for biofilm formation studies by Staphylococcus aureus is demonstrated. Laboratory-coated papers with different physicochemical properties were used as substrates. The array platform was fabricated by patterning the coated papers with vinyl-substituted polydimethylsiloxane (PDMS) -based ink. The affinity of bacteria onto the flexographically printed hydrophobic and smooth PDMS film was very low whereas bacterial adhesion and biofilm formation occurred preferentially on the unprinted areas, i.e. in the reaction arrays. The concentration of the attached bacteria was quantified by determining the viable colony forming unit (CFU/cm²) numbers. The distribution and the extent of surface coverage of the biofilms were determined by atomic force microscopy. In static conditions, the highest bacterial concentration and most highly organized biofilms were observed on substrates with high polarity. On a rough paper surface with low polarity, the biofilm formation was most hindered. Biofilms were effectively removed from a polar substrate upon exposure to (+)-dehydroabietic acid, an anti-biofilm compound.

Bacteriophage-aided intracellular killing of engulfed methicillin-resistant Staphylococcus aureus (MRSA) by murine macrophages

Phages are known to effectively kill extracellularly multiplying bacteria as they do not have the ability of intracellular penetration within the animal cells. However, the present manuscript focuses on studying the impact of surface-adsorbed phage particles on the killing of engulfed Staphylococcus aureus inside phagocytic cells. Mouse peritoneal macrophages were isolated and cultured, followed by evaluation of their ability of bacterial uptake and killing. The intracellular killing potential of macrophages in the presence of unadsorbed free phage as well as phage adsorbed onto S. aureus 43300 was studied. Phage added alone to macrophage preparation did not influence intracellular killing of engulfed S. aureus by macrophages. However, phage adsorbed onto host bacterial cells (utilizing host bacteria as a vehicle to carry the lytic phage into the phagocytic compartment) brought about time-dependent and titre-dependent significant reduction in the number of viable intracellular cocci. Phage particles that shuttled inside the macrophage along with bacteria also significantly reduced cytotoxic damage caused by methicillin-resistant S. aureus (MRSA). This in turn enhanced the bactericidal killing potential of phagocytic cells. In earlier studies the inability of phages to kill intracellular bacteria has been thought to be a major drawback of phage therapy. For the first time results of this study confirm the killing ability of the broad host range lytic phage MR-5 of both extracellular as well as intracellular engulfed S. aureus inside macrophages. This approach shall not only restrict intracellular proliferation of staphylococci within the myeloid cells but also protect the host from further relapse of infection and treatment failures.

PEGylated liposome encapsulation increases the lung tissue concentration of vancomycin

Pneumonia due to methicillin-resistant Staphylococcus aureus (MRSA) often cannot be cured by vancomycin treatment. Poor lung tissue and intracellular penetration limits the ability to achieve effective bactericidal levels, particularly in alveolar macrophages, where MRSA can evade phagocytic killing. Compared to standard formulations, liposome encapsulation has been shown to enhance vancomycin intracellular killing of MRSA. In this murine pharmacokinetic and biodistribution study, PEGylated liposomal vancomycin, compared to standard and non-PEGylated formulations, significantly prolonged blood circulation time and increased deposition in lung, liver, and spleen and yet reduced accumulation in kidney tissue. As a result of optimizing antimicrobial targeting of infected lung tissue and limiting renal parenchymal exposure, administration of PEGylated liposomal vancomycin may improve the efficacy of treatment of MRSA pneumonia and reduce the risk of nephrotoxicity.

Impact of burden on granulocyte clearance of bacteria in a mouse thigh infection model

We wished to delineate granulocytes' impact on the clearance of different bacterial burdens of Pseudomonas aeruginosa and Staphylococcus aureus in a granulocyte-replete mouse thigh infection model. A mouse thigh model was employed. Bacterial challenges from 10(5) to 3 × 10(7) CFU (S. aureus) and from 3 × 10(4) to 3 × 10(8) CFU (P. aeruginosa) were injected into murine posterior thighs. Organism quantitation was at baseline, 2 h (Pseudomonas only), and 24 h. A Michaelis-Menten population model was fit to the data for each organism. Breakpoints for microbial containment by granulocytes were identified. Bacterial burdens exceeding that breakpoint value resulted in organism multiplication. The Michaelis-Menten model fit the data well. For P. aeruginosa, the observed-predicted plot had a regression equation that explained over 98% of the variance (P ≪ 0.001). For S. aureus, this relationship explained greater than 94% of the variance (P ≪ 0.001). Maximal growth rate constants, maximal population burdens, and the bacterial loads at which granulocytes killed if half-saturated were not different. The kill rate constant for P. aeruginosa was almost 10 times that of S. aureus. Bacterial kill by granulocytes is saturable. No difference between saturation points of different isolates was seen. A higher bacterial burden means an increasing reliance on chemotherapy to drive bacterial clearance.

Intra- and extracellular activities of dicloxacillin against Staphylococcus aureus in vivo and in vitro

Antibiotic treatment of Staphylococcus aureus infections is often problematic due to the slow response and recurrences. The intracellular persistence of the staphylococci offers a plausible explanation for the treatment difficulties because of the impaired intracellular efficacies of the antibiotics. The intra- and extracellular time- and concentration-kill relationships were examined in vitro with THP-1 cells and in vivo by use of a mouse peritonitis model. The in vivo model was further used to estimate the most predictive pharmacokinetic/pharmacodynamic (PK/PD) indices (the ratio of the maximum concentration of drug in plasma/MIC, the ratio of the area under the concentration-time curve/MIC, or the cumulative percentage of a 24-h period that the free [f] drug concentration exceeded the MIC under steady-state pharmacokinetic conditions [fT(MIC)]) for dicloxacillin (DCX) intra- and extracellularly. In general, DCX was found to have similar intracellular activities, regardless of the model used. Both models showed (i) the relative maximal efficacy (1-log-unit reduction in the numbers of CFU) of DCX intracellularly and (ii) the equal relative potency of DCX intra- and extracellularly, with the MIC being a good indicator of the overall response in both situations. Discordant results, based on data obtained different times after dosing, were obtained from the two models when the extracellular activity of DCX was measured, in which the in vitro model showed a considerable reduction in the number of CFU from that in the original inoculum (3-log-unit decrease in the number of CFU after 24 h), whereas the extracellular CFU reduction achieved in vivo after 4 h did not exceed 1 log unit. Multiple dosing of DCX in vivo revealed increased extra- and intracellular efficacies (2.5 log and 2 log units of reduction in the numbers of CFU after 24 h, respectively), confirming that DCX is a highly active antistaphylococcal antibiotic. PK/PD analysis revealed that fT(MIC) is the index that is the most predictive of the outcome of infection both intra- and extracellularly.

Intra- and extracellular activity of linezolid against Staphylococcus aureus in vivo and in vitro

BACKGROUND AND AIMS

Treatment of Staphylococcus aureus infections remains problematic (slow responses and frequent recurrences). Intracellular persistence of the S. aureus could explain those difficulties because of impaired intracellular efficacy of antibiotics. Our aim was to study linezolid for its intracellular activity.

METHODS

(i) Pharmacodynamic (PD) analysis of intracellular activity using in vitro (THP-1 macrophages) and in vivo (mouse peritonitis) models with determination of key dose-response parameters [maximal relative efficacy (E(max)), relative potency (EC(50)) and static concentration (C(static))] towards methicillin-susceptible S. aureus (ATCC 25923; clinical isolate) with linezolid MICs of 4 mg/L; (ii) pharmacokinetic (PK) analysis in uninfected mice for determination of C(max), AUC and half-life for total and free drug; and (iii) determination of the predictive PK/PD parameter (fT > MIC, fAUC(24)/MIC or fC(max)/MIC) for therapeutic outcome.

RESULTS

In vitro, linezolid showed an E(max) of approximately 1 log(10) cfu reduction compared with initial inoculum both intra- and extracellularly and an approximately 3-fold increased relative potency (lower EC(50) and C(static)) intracellularly. In vivo, the efficacy of linezolid was impaired (<0.5 log(10) reduction extracellularly; failure to reduce the cfu to less than the initial load intracellularly) with, however, an increased intracellular potency (lower EC(50)). Infection outcome correlated better with the fAUC(24)/MIC (R(2) = 55%) than with the fT > MIC parameter (R(2) = 51%) for the extracellular compartment, but no parameter emerged as significant for the intracellular compartment.

CONCLUSIONS

Linezolid exerts only a weak intracellular activity against the strains of S. aureus tested, even though, in contrast to most other antibiotics, its potency does not appear impaired in comparison with the extracellular activity.

Staphylococcal PknB as the first prokaryotic representative of the proline-directed kinases

In eukaryotic cell types, virtually all cellular processes are under control of proline-directed kinases and especially MAP kinases. Serine/threonine kinases in general were originally considered as a eukaryote-specific enzyme family. However, recent studies have revealed that orthologues of eukaryotic serine/threonine kinases exist in bacteria. Moreover, various pathogenic species, such as Yersinia and Mycobacterium, require serine/threonine kinases for successful invasion of human host cells. The substrates targeted by bacterial serine/threonine kinases have remained largely unknown. Here we report that the serine/threonine kinase PknB from the important pathogen Staphylococcus aureus is released into the external milieu, which opens up the possibility that PknB does not only phosphorylate bacterial proteins but also proteins of the human host. To identify possible human targets of purified PknB, we studied in vitro phosphorylation of peptide microarrays and detected 68 possible human targets for phosphorylation. These results show that PknB is a proline-directed kinase with MAP kinase-like enzymatic activity. As the potential cellular targets for PknB are involved in apoptosis, immune responses, transport, and metabolism, PknB secretion may help the bacterium to evade intracellular killing and facilitate its growth. In apparent agreement with this notion, phosphorylation of the host-cell response coordinating transcription factor ATF-2 by PknB was confirmed by mass spectrometry. Taken together, our results identify PknB as the first prokaryotic representative of the proline-directed kinase/MAP kinase family of enzymes.

Staphylococcal catalase protects intracellularly survived bacteria by destroying H2O2 produced by the murine peritoneal macrophages

To determine the interrelationship between the hydrogen peroxide (H(2)O(2)) mediated killing and the potential role of bacterial catalase and SOD in the evasion of host defense, we examined three clinical isolates of Staphylococcus aureus and evaluated their intracellular survival mechanism within murine peritoneal macrophages. Fluorescent microscopy and bacterial colony-forming unit (cfu) count revealed that phagocytic capacity of murine peritoneal macrophages was highest after 2h of in vitro infection with S. aureus. To understand whether catalase and SOD contributing in the intracellular survival, were of bacterial origin or not, 3 amino 1,2,4 triazole (ATZ) and Diethyldithiocarbamic acid (DDC) were used to inhibit specifically macrophage derived catalase and SOD respectively. Catalase activity from the whole staphylococcal cell in presence of ATZ suggested that the released catalase were of extracellular origin. Scanning electron microscopy revealed the degraded host cell membrane integrity during prolonged infection. Purified bacterial catalase from the intracellularly survived S. aureus recovered after 5h of infection and its inhibition by ATZ in the zymography strengthened the scope of involvement of these anti-oxidants in the intracellular survival of S. aureus.

Intracellular activity of antibiotics against Staphylococcus aureus in a mouse peritonitis model

Antibiotic treatment of Staphylococcus aureus infections is often problematic due to the slow response to therapy and the high frequency of infection recurrence. The intracellular persistence of staphylococci has been recognized and could offer a good explanation for these treatment difficulties. Knowledge of the interplay between intracellular antibiotic activity and the overall outcome of infection is therefore important. Several intracellular in vitro models have been developed, but few experimental animal models have been published. The mouse peritonitis/sepsis model was used as the basic in vivo model exploring a quantitative ex vivo extra- and intracellular differentiation assay. The intracellular presence of S. aureus was documented by electron microscopy. Five antibiotics, dicloxacillin, cefuroxime, gentamicin, azithromycin, and rifampin (rifampicin), were tested in the new in vivo model; and the model was able to distinguish between their extra- and intracellular effects. The intracellular effects of the five antibiotics could be ranked as follows as the mean change in the log(10) number of CFU/ml (Delta log(10) CFU/ml) between treated and untreated mice after 4 h of treatment: dicloxacillin (3.70 Delta log(10) CFU/ml) > cefuroxime (3.56 Delta log(10) CFU/ml) > rifampin (1.86 Delta log(10) CFU/ml) > gentamicin (0.61 Delta log(10) CFU/ml) > azithromycin (0.21 Delta log(10) CFU/ml). We could also show that the important factors during testing of intracellular activity in vivo are the size, number, and frequency of doses; the time of exposure; and the timing between the start of infection and treatment. A poor correlation between the intracellular accumulation of the antibiotics and the actual intracellular effect was found. This stresses the importance of performing experimental studies, like those with the new in vivo model described here, to measure actual intracellular activity instead of making predictions based on cellular pharmacokinetic and MICs.

A potential new pathway for Staphylococcus aureus dissemination: the silent survival of S. aureus phagocytosed by human monocyte-derived macrophages

Although considered to be an extracellular pathogen, Staphylococcus aureus is able to invade a variety of mammalian, non-professional phagocytes and can also survive engulfment by professional phagocytes such as neutrophils and monocytes. In both of these cell types S. aureus promptly escapes from the endosomes/phagosomes and proliferates within the cytoplasm, which quickly leads to host cell death. In this report we show that S. aureus interacted with human monocyte-derived macrophages in a very different way to those of other mammalian cells. Upon phagocytosis by macrophages, S. aureus persisted intracellularly in vacuoles for 3-4 days before escaping into the cytoplasm and causing host cell lysis. Until the point of host cell lysis the infected macrophages showed no signs of apoptosis or necrosis and were functional. They were able to eliminate intracellular staphylococci if prestimulated with interferon-gamma at concentrations equivalent to human therapeutic doses. S. aureus survival was dependent on the alternative sigma factor B as well as the global regulator agr, but not SarA. Furthermore, isogenic mutants deficient in alpha-toxin, the metalloprotease aureolysin, protein A, and sortase A were efficiently killed by macrophages upon phagocytosis, although with different kinetics. In particular alpha-toxin was a key effector molecule that was essential for S. aureus intracellular survival in macrophages. Together, our data indicate that the ability of S. aureus to survive phagocytosis by macrophages is determined by multiple virulence factors in a way that differs considerably from its interactions with other cell types. S. aureus persists inside macrophages for several days without affecting the viability of these mobile cells which may serve as vehicles for the dissemination of infection.

PerR controls oxidative stress resistance and iron storage proteins and is required for virulence in Staphylococcus aureus

The Staphylococcus aureus genome encodes three ferric uptake regulator (Fur) homologues: Fur, PerR, and Zur. To determine the exact role of PerR, we inactivated the gene by allelic replacement using a kanamycin cassette, creating strain MJH001 (perR). PerR was found to control transcription of the genes encoding the oxidative stress resistance proteins catalase (KatA), alkyl hydroperoxide reductase (AhpCF), bacterioferritin comigratory protein (Bcp), and thioredoxin reductase (TrxB). Furthermore, PerR regulates transcription of the genes encoding the iron storage proteins ferritin (Ftn) and the ferritin-like Dps homologue, MrgA. Transcription of perR was autoregulated, and PerR repressed transcription of the iron homeostasis regulator Fur, which is a positive regulator of catalase expression. PerR functions as a manganese-dependent, transcriptional repressor of the identified regulon. Elevated iron concentrations produced induction of the PerR regulon. PerR may act as a peroxide sensor, since addition of external hydrogen peroxide to 8325-4 (wild type) resulted in increased transcription of most of the PerR regulon, except for fur and perR itself. The PerR-regulated katA gene encodes the sole catalase of S. aureus, which is an important starvation survival determinant but is surprisingly not required for pathogenicity in a murine skin abscess model of infection. In contrast, PerR is not necessary for starvation survival but is required for full virulence (P < 0.005) in this model of infection. PerR of S. aureus may act as a redox sentinel protein during infection, analogous to the in vitro activities of OxyR and PerR of Escherichia coli and Bacillus subtilis, respectively. However, it differs in its response to the metal balance within the cell and has the added capability of regulating iron uptake and storage.

Effect of treatment with methicillin and gentamicin in a new experimental mouse model of foreign body infection

A new mouse model of foreign body infection has been developed. Intraperitoneal placement of a silicone catheter followed by injection of 10(8) Staphylococcus aureus organisms resulted in a reproducible, localized foreign body infection. The infection persisted as an intra-abdominal abscess surrounding the catheter for at least 30 days. Treatment with up to nine doses of methicillin or gentamicin or both was started 3 days after infection. The treatment showed a significant effect (P < 0.05), measured as reduction of bacteria on the foreign body, for all three regimens with a reduction of up to 2 log units, but no synergism was observed. The result of the treatment was poor, despite the facts that the local concentrations of methicillin were greater than the MIC for at least 72 h and that nine peak concentrations of gentamicin of > 13 micrograms/ml were obtained. The poor result of the treatment was not caused by development of antibiotic resistance or influenced by protein concentration, pH, or local presence in the pus of inhibitors of antibiotics. Both antibiotics showed good effects in time-kill studies in vitro on bacteria on catheters taken out of infected mice and catheters infected in vitro. During treatment, the proportion of intracellular bacteria increased in all treated mice to 60 to 75% compared with 20 to 30% in nontreated mice (P < 0.05). This indicates that intracellular survival of staphylococci may influence the outcome of the treatment in foreign body infections.

Efficacies of various antimicrobial agents in treatment of Staphylococcus aureus abscesses and correlation with in vitro tests of antimicrobial activity and neutrophil killing

A rabbit perforated-capsule model was utilized to study antimicrobial efficacy in treating 2-week-old Staphylococcus aureus abscesses. Animals received either ciprofloxacin (30 mg/kg), cefazolin (100 mg/kg), or ciprofloxacin (30 mg/kg) plus rifampin (20 mg/kg) every 8 h for 8 days or no antibiotic. Antibiotic levels within the abscess exceeded the MIC for the test organism. At the end of treatment, ciprofloxacin was no more effective than the control, animals receiving cefazolin had a mean log10 fall of 2.41 CFU/ml, and animals receiving ciprofloxacin plus rifampin had a mean log10 reduction of 5.06 CFU/ml (P = less than 0.01). Six days after completion of therapy, all abscesses in animals receiving ciprofloxacin plus rifampin were culture negative. Surviving organisms in animals receiving ciprofloxacin or rifampin did not develop resistance to the treatment antibiotics. In vitro time-kill curves performed with logarithmic- and stationary-phase organisms in broth, serum, and abscess fluid supernatants did not correlate with the in vivo results. Neutrophil killing studies of S. aureus pretreated with antibiotics revealed greater killing of organisms pretreated with ciprofloxacin plus rifampin than of those pretreated with cefazolin or ciprofloxacin alone. In conclusion, ciprofloxacin plus rifampin was effective therapy in this staphylococcal abscess model, compared with the moderate efficacy of cefazolin and no effect observed with ciprofloxacin alone. Enhanced neutrophil killing of S. aureus pretreated with antibiotics may be an important mechanism by which bacteria are killed in suppurative infections.

The effect of antistaphylococcal agents used alone and in combinations on the survival of Staphylococcus aureus ingested by human polymorphonuclear leukocytes

The intracellular activity of a number of drugs used alone and in combinations against Staphylococcus aureus was investigated using an experimental design which imitates the clinical situation and differs from other published methods. Staphylococci were phagocytosed by human polymorphonuclear leukocytes and, after differential centrifugation and washing, the granulocytes were incubated in 90% pooled human serum with clinically relevant drug concentrations. When exposed to antibiotics, more than 40-50% of the bacteria were located intracellularly. Fusidic acid (100 mg/l), erythromycin (20 mg/l), and clindamycin (20 mg/l) all had a bacteriostatic effect during the first 6 h of incubation, whereas rifampicin (1 and 5 mg/l), vancomycin (5 and 20 mg/l), and ciprofloxacin (2 mg/l) all acted bactericidally with decreases in viable counts between 1.3-1.9 log10. The greatest bactericidal effect was achieved with tobramycin (10 mg/l), which produced more than a 4 log10 decrease in viable counts at 6 h. Combinations of fusidic acid with other antibiotics all resulted in killing kinetics different from those achieved with the drugs used individually. The bactericidal effect of ciprofloxacin and dicloxacillin during the first 6 h was abolished when these drugs were combined with fusidic acid. However, at 24 h no significant difference was found between the effect of dicloxacillin alone versus the combination dicloxacillin and fusidic acid. The combination of fusidic acid and rifampicin resulted in a killing identical to that achieved with rifampicin used alone during the first 6 h, but at 24 h the killing by the combination was significantly greater. The bactericidal effect of the combination dicloxacillin (20 mg/l) and tobramycin (10 mg/l) equalled that obtained with tobramycin (10 mg/l) used alone. Rifampicin (5 mg/l) antagonized the bactericidal effect of ciprofloxacin (2 mg/l) during the first 6 h of incubation but at 24 h the combination acted synergistically. The results obtained are partly in agreement and partly in conflict with previous results.

Activity of antibiotics against microorganisms ingested by mononuclear phagocytes

The data available on the activity of antibiotics against microorganisms ingested by mononuclear phagocytes are reviewed. The antibacterial activity of penicillins against Staphylococcus aureus is enhanced by human monocytes. This enhancement is possibly brought about by a peptidoglycan-degrading enzyme that is produced and secreted by monocytes. Aminoglycosides affect bacteria ingested by mononuclear phagocytes, but their intracellular activity is limited as compared with that of non-phagocytosed bacteria. Clindamycin, erythromycin, ciprofloxacin and rifampicin are effective against intracellular bacteria. The first three of these antibiotics do not show enhanced activity against phagocytosed bacteria, contrary to what might be expected from their high cell-associated concentrations.

Effect of amoxycillin and clavulanic acid, alone and in combination, on human polymorphonuclear leukocyte function against Staphylococcus aureus

The effect of amoxycillin and clavulanic acid on the interaction in vitro of human polymorphonuclear leukocytes with Staphylococcus aureus was examined. The exposure of a non-penicillinase producing Staphylococcus aureus strain to one-fourth the MIC of amoxycillin or clavulanic acid alone significantly increased the uptake of both unopsonized and opsonized bacteria by human polymorphonuclear leukocytes. This effect was also observed when bacteria were exposed to one-fourth the MIC of different proportions of the combination of amoxycillin and clavulanic acid (4/1, 1/1, 1/8 and 1/32). When a penicillinase-producing Staphylococcus aureus strain was used, only clavulanic acid significantly increased the uptake of unopsonized bacteria. The production of superoxide radicals by human polymorphonuclear leukocytes was impaired only by the presence of high concentrations (100 mg/l) of both clavulanic acid or the combination amoxycillin/clavulanic acid (4/1). At this high concentration, however, amoxycillin/clavulanic acid (4/1) significantly increased the intracellular killing of Staphylococcus aureus.

Effect of clindamycin on neutrophil killing of gram-negative periodontal bacteria

Periodontal diseases are infections of the tissues supporting the dentition. Recognition that relatively specific microfloras are associated with distinct clinical forms of periodontal disease has prompted the use of antimicrobial agents as adjuncts in periodontal therapy. Clindamycin is one of several antibiotics known to concentrate in bioactive form in neutrophils and to potentiate phagocyte bactericidal activity against certain bacteria. Neutrophils appear to play a key role in host defense against periodontopathic gram-negative bacteria. In the present study, we evaluated the effect of preincubation of neutrophils with therapeutically achievable concentrations of clindamycin upon subsequent in vitro bactericidal activity against three species of gram-negative periodontal bacteria, including Actinobacillus actinomycetemcomitans, Eikenella corrodens, and Capnocytophaga ochracea. In each instance, clindamycin neither enhanced nor inhibited the kinetics of bactericidal activity at low bacterium-neutrophil multiplicities. Further, this antibiotic had no demonstrable effect upon neutrophil bactericidal capacity, as assessed at bacterium-neutrophil ratios as high as 50:1. Our results indicate that clindamycin does not potentiate neutrophil bactericidal activity against the species of gram-negative periodontal organisms tested.

Enhanced phagocytosis, killing, and serum sensitivity of Escherichia coli and Staphylococcus aureus treated with sub-MICs of imipenem

The influence of pretreatment of Escherichia coli and Staphylococcus aureus with sub-MICs of the new beta-lactam antibiotic imipenem on phagocytosis and killing by murine peritoneal macrophages and the susceptibility of these organisms to serum bactericidal activity were studied. The effects of imipenem, a round form inducer in gram-negative rods, and piperacillin, a filamentous form inducer, were compared. Bacteria grown in the presence of sub-MICs of imipenem or piperacillin were incubated for 30 min with macrophage monolayers in the absence of antibiotic. Phagocytosis, killing, and survival within macrophages were evaluated by microbiological and fluorescence microscope assays. Bacteria grown in the presence of a sub-MIC of imipenem were phagocytized and killed in numbers significantly higher than untreated or piperacillin-treated bacteria were. Intracellular bacteria pretreated with a sub-MIC of imipenem were also readily killed by lymphokine-activated macrophages. Prior treatment with a sub-MIC of imipenem resulted in an increased susceptibility of E. coli but not S. aureus to the bactericidal activity of immune serum. Imipenem treatment and immune serum acted synergistically to enhance phagocytosis and killing. The data indicate that exposure of E. coli and S. aureus to a sub-MIC of imipenem enhances the susceptibility of these potential pathogens to cellular and humoral host defense mechanisms.

Interaction of beta-lactam antibiotics with the bactericidal activity of leukocytes against Escherichia coli

The effect of beta-lactam antibiotics on phagocytosis and intracellular killing of four isogenic Escherichia coli strains differing in their 0- and K antigens was studied by adopting the rat polyvinyl-sponge model. The penicillins mezlocillin, ticarcillin and piperacillin rendered all four isogenic E. coli strains more susceptible to intraleukocyte killing; the cefalosporins tested exhibited inhomogenous effects; lamoxactam was marginally effective, whereas cefoxitin was completely ineffective; cefotaxime caused an increase in intracellular killing of the capsule-defective mutant only. The beta-lactam promoted increase in intracellular killing could be inhibited by alpha-methylmannoside but not by alpha-methylglucoside. Free-flow electrophoretic separation of mezlocillin-treated bacteria and guinea pig erythrocytes revealed that co-migration of E. coli and erythrocytes respectively could be inhibited by alpha-methylmannoside but not by alpha-methylglucoside. These data indicate that mezlocillin interferes with the mannose sensitive adhesins of E. coli.

Effect of ciprofloxacin on phagocytosis

Certain aspects of the relationship between host defence mechanisms and the new quinoline derivative ciprofloxacin in comparison to norfloxacin and ofloxacin were studied. Ciprofloxacin did not affect chemotaxis of human polymorphonuclear leucocytes in agarose. In leucocytes exposed to ciprofloxacin, norfloxacin and ofloxacin neither the chemiluminescent response to opsonized zymosan and formyl-methionyl-leucyl-phenylalanine nor the phagocytic or bactericidal activity was affected. However, killing of Staphylococcus aureus by human polymorphonuclear leucocytes exposed to subinhibitory concentrations of ciprofloxacin was enhanced. The results show that the quinolines tested do not directly influence phagocytic cells, but a subinhibitory concentration can make bacteria more susceptible to phagocytosis and killing.

Adherence of lysostaphin to and penetration into human monocytes

The effect of lysostaphin on Staphylococcus aureus phagocytosed by monocytes was investigated. The results showed that lysostaphin adheres to monocytes by a temperature-independent mechanism, is not adequately removed from monocytes by washing, and penetrates by means of a temperature-dependent mechanism. In in vitro assays of monocyte function, phagocytosed S. aureus can be killed by lysostaphin after penetration of the cells during incubation or by adhering lysostaphin when the monocytes are disrupted.

Therapeutic relevance of penicillin-induced hypersensitivity of Staphylococcus aureus to killing by polymorphonuclear leukocytes

There is an overwhelming body of evidence that certain Staphylococcus aureus strains become more sensitive to killing by polymorphonuclear leukocytes after their growth in media containing subinhibitory concentrations of penicillin. However, it is not clear to what extent this phenomenon contributes to the curative effect of penicillin in vivo. To explore its therapeutic relevance, we evaluated the interaction of staphylococci pretreated with penicillin in vitro with leukocytes in cell-proof diffusion chambers (porosity, 0.22 micron) implanted subcutaneously in rabbits. Under this in vivo environment, staphylococci pretreated with penicillin remained hypersensitive to leukocyte killing as under in vitro conditions. Furthermore, when the staphylococci were mixed with the leukocytes in chambers implanted intraperitoneally in mice which subsequently received intravenously a suboptimal dose of penicillin, they also became hypersensitive to leukocytic killing. However, because the staphylococcal growth rate was considerably reduced in vivo, the degree of penicillin-induced sensitivity to leukocytic killing was smaller than that obtained in test tube cultures; nevertheless, the enhanced killing was significant. Additional support that the curative effect of penicillin partly depends on its synergistic action with the leukocytes was provided by the relative decrease in virulence of staphylococci pretreated with penicillin in mice in which the cellular host defenses were already recruited at the focus of inoculation. These observations indicate that penicillin-induced hypersensitivity of staphylococci to leukocytic killing is not only an in vitro phenomenon, but an effect which has therapeutic relevance.