Quantitative studies of nasal carriers of staphylococci among hospitalized patients

Staphylococcus aureus colonisation and strategies for decolonisation

Staphylococcus aureus is a leading cause of death by infectious diseases worldwide. Treatment of S aureus infections is difficult due to widespread antibiotic resistance, necessitating alternative approaches and measures for prevention of infection. Because S aureus infections commonly arise from asymptomatic colonisation, decolonisation is considered a key approach for their prevention. Current decolonisation procedures include antibiotic-based and antiseptic-based eradication of S aureus from the nose and skin. However, despite the widespread implementation and partial success of such measures, S aureus infection rates remain worrisome, and resistance to decolonisation agents is on the rise. In this Review we outline the epidemiology and mechanisms of S aureus colonisation, describe how colonisation underlies infection, and discuss current and novel approaches for S aureus decolonisation, with a focus on the latest findings on probiotic strategies and the intestinal S aureus colonisation site.

SaQuant: a real-time PCR assay for quantitative assessment of Staphylococcus aureus

Background

Molecular assays are important tools for pathogen detection but need to be periodically re-evaluated with the discovery of additional genetic diversity that may cause assays to exclude target taxa or include non-target taxa. A single well-developed assay can find broad application across research, clinical, and industrial settings. Pathogen prevalence within a population is estimated using such assays and accurate results are critical for formulating effective public health policies and guiding future research. A variety of assays for the detection of Staphylococcus aureus are currently available. The utility of commercial assays for research is limited, given proprietary signatures and lack of transparent validation.

Results

In silico testing of existing peer-reviewed assays show that most suffer from a lack of sensitivity and specificity. We found no assays that were specifically designed and validated for quantitative use. Here we present a qPCR assay, SaQuant, for the detection and quantification of S. aureus as might be collected on sampling swabs. Sensitivity and specificity of the assay was 95.6 and 99.9 %, respectively, with a limit of detection of between 3 and 5 genome equivalents and a limit of quantification of 8.27 genome equivalents. The presence of DNA from non-target species likely to be found in a swab sample, did not impact qualitative or quantitative abilities of the assay.

Conclusions

This assay has the potential to serve as a valuable tool for the accurate detection and quantification of S. aureus collected from human body sites in order to better understand the dynamics of prevalence and transmission in community settings.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02247-6.

Rapid cell division of Staphylococcus aureus during colonization of the human nose

BACKGROUND

Staphylococcus aureus is an important opportunistic pathogen and a commensal bacterium, thriving in the nasal cavities of 20% of the human population. Little is known about the dynamics of asymptomatic colonization and the occasional transition to infectious disease.

RESULTS

In this study, we inferred that S. aureus cells replicate every one to three hours on average while colonizing the human nose, based on two independent lines of genomic evidence. First, we collected nasal swab samples from human subjects, extracted and sequenced metagenomic DNA, and analyzed the distribution of sequencing coverage along the staphylococcal chromosome. Calibration of this data by comparison to a laboratory culture enabled measuring S. aureus cell division rates in nasal samples. Second, we applied mutation accumulation experiments paired with genome sequencing to measure spontaneous mutation rates at a genome scale. Relating these mutation rates to annual evolutionary rates confirmed that nasal S. aureus continuously pass several thousand cell divisions per year when averaged over large, globally distributed populations and over many years, corresponding to generation times of less than two hours.

CONCLUSIONS

The cell division rates we determined were higher than the fastest documented rates during fulminant disease progression (in a mouse model of systemic infection) and much higher than those previously measured in expectorated sputum from cystic fibrosis patients. This paper supplies absolute in-vivo generation times for an important bacterial commensal, indicating that colonization of the human upper respiratory tract is characterized by a highly dynamic equilibrium between bacterial growth and removal.

Staphylococcus aureus Colonization: Modulation of Host Immune Response and Impact on Human Vaccine Design

In apparent contrast to its invasive potential Staphylococcus aureus colonizes the anterior nares of 20–80% of the human population. The relationship between host and microbe appears particularly individualized and colonization status seems somehow predetermined. After decolonization, persistent carriers often become re-colonized with their prior S. aureus strain, whereas non-carriers resist experimental colonization. Efforts to identify factors facilitating colonization have thus far largely focused on the microorganism rather than on the human host. The host responds to S. aureus nasal colonization via local expression of anti-microbial peptides, lipids, and cytokines. Interplay with the co-existing microbiota also influences colonization and immune regulation. Transient or persistent S. aureus colonization induces specific systemic immune responses. Humoral responses are the most studied of these and little is known of cellular responses induced by colonization. Intriguingly, colonized patients who develop bacteremia may have a lower S. aureus-attributable mortality than their non-colonized counterparts. This could imply a staphylococcal-specific immune “priming” or immunomodulation occurring as a consequence of colonization and impacting on the outcome of infection. This has yet to be fully explored. An effective vaccine remains elusive. Anti-S. aureus vaccine strategies may need to drive both humoral and cellular immune responses to confer efficient protection. Understanding the influence of colonization on adaptive response is essential to intelligent vaccine design, and may determine the efficacy of vaccine-mediated immunity. Clinical trials should consider colonization status and the resulting impact of this on individual patient responses. We urgently need an increased appreciation of colonization and its modulation of host immunity.

MRSA nasal colonization burden and risk of MRSA infection

BACKGROUND

Staphylococcus aureus nasal colonization burden has been identified as a risk factor for infection. This study evaluates methicillin-resistant S aureus (MRSA) nasal burden, as defined by the cycle threshold (Ct) and risk of subsequent infection.

METHODS

In a retrospective cohort study, United States veterans were classified into 3 MRSA nasal colonization groups: noncarriers, low burden (Ct > 24 cycles), and high burden (Ct ≤ 24 cycles). MRSA infections were identified prospectively, and clinical information was obtained by chart review. Multivariate logistic regression assessed the association of MRSA nasal burden and risk of MRSA infection.

RESULTS

During 4-years of follow-up, 4.3% of noncarriers, 18.5% of low burden, and 17.2% of high burden developed a MRSA infection. In multivariate analysis, MRSA nasal colonization was a risk factor for MRSA infection (P = .008) with low burden (risk ratio [RR], 3.62; 95% confidence interval [CI]: 1.47-8.93) and high burden (RR, 2.71; 95% CI: 0.95-7.72) associated with subsequent MRSA infection when compared with noncarriers. When compared with low burden, high burden nasal carriers were not at increased risk of infection (RR, 0.75; 95% CI 0.36-1.55).

CONCLUSION

MRSA nasal colonization was a risk factor for MRSA infection. High nasal burden of MRSA did not increase the risk of infection.

Impaired β-defensin expression in human skin links DEFB1 promoter polymorphisms with persistent Staphylococcus aureus nasal carriage

BACKGROUND

Genetically determined variation in the expression of innate defense molecules may explain differences in the propensity to be colonized with Staphylococcus aureus.

METHODS

We determined S. aureus nasal carriage in 603 volunteers; analyzed polymorphisms in the DEFB1 promoter at positions -52 G>A (rs1799946), -44 C>G (rs1800972), and -20 G>A (rs11362); and measured the content of human β-defensin 1 (hBD-1) and hBD-3 messenger RNA (mRNA) in 192 samples of healthy and experimentally wounded human skin.

RESULTS

Compared with GGG at the positions -52/-44/-20, the ACG haplotype was more common among persistent S. aureus nasal carriers (odds ratio, 1.93; 95% confidence interval [CI], 1.2-3.1; P = .006) and was associated with reduced expression of hBD-1 (GGG>ACG>GCA; P < .001) and hBD-3 (GGG>GCA>ACG; P = .04) in skin when measured 72 hours after wounding. Furthermore, a 50% decrease in hBD-1 and hBD-3 mRNA expression in wounded skin increased the odds of persistent carriage by 1.45 (95% CI, .93-2.26; P = .1) and 1.48 (95% CI, 1.01-2.17; P = .04), respectively. Adjustment for known risk factors of persistent S. aureus carriage did not substantially change the associations of both DEFB1 haplotypes and β-defensin expression with S. aureus colonization.

CONCLUSIONS

DEFB1 polymorphisms may promote persistent S. aureus colonization by altering β-defensin expression in keratinocytes of human skin.

Hormonal contraceptive use and persistent Staphylococcus aureus nasal carriage

BACKGROUND

Human nares colonized with Staphylococcus aureus are the most important reservoir for this pathogen. We studied the influence of sex and hormonal contraceptive use on persistent S. aureus nasal carriage.

METHODS

We conducted a cohort study in healthy volunteers and determined carriage status at baseline and again at follow-up by using the results of 2 swab samples at each time point. We applied logistic regression to analyze associations of interest.

RESULTS

At baseline, 266 of 1180 volunteers (22.5%) were classified as persistent nasal carriers. Compared with women not using hormonal contraceptives, women taking reproductive hormones (odds ratio [OR]. 1.88; 95% confidence interval [CI], 1.29-2.75; P = .001) and men (OR., 1.57; 95% CI, 1.08-2.28; P = .02) were more likely to be persistent carriers. These associations remained stable after adjusting for known risk factors of nasal carriage. Women taking hormonal contraceptives and being persistent carriers at baseline were more likely to remain carriers after a median follow-up time of 70 days than women not using such medication (OR, 3.25; 95% CI, 1.44-7.34; P = .005). No patterns of association could be observed between persistent carriage among women and type of progestin or dose of estrogen used. Assuming causality and using estimates from multivariable logistic regression, we approximated that 20% (95% CI, 2.4%-34.9%) of persistent nasal carriage among women represented by our sample is attributable to hormonal contraception (population-attributable fraction).

CONCLUSIONS

The widespread use of hormonal contraception may substantially increase the human S. aureus reservoir with potential impact on S. aureus infection and transmission.

Direct detection of nasal Staphylococcus aureus carriage via helicase-dependent isothermal amplification and chip hybridization

BACKGROUND

The bacterium Staphylococcus aureus constitutes one of the most important causes of nosocomial infections. One out of every three individuals naturally carries S. aureus in their anterior nares, and nasal carriage is associated with a significantly higher infection rate in hospital settings. Nasal carriage can be either persistent or intermittent, and it is the persistent carriers who, as a group, are at the highest risk of infection and who have the highest nasal S. aureus cell counts. Prophylactic decolonization of S. aureus from patients' noses is known to reduce the incidence of postsurgical infections, and there is a clear rationale for rapid identification of nasal S. aureus carriers among hospital patients.

FINDINGS

A molecular diagnostic assay was developed which is based on helicase-dependent target amplification and amplicon detection by chip hybridization to a chip surface, producing a visible readout. Nasal swabs from 70 subjects were used to compare the molecular assay against culturing on "CHROMagar Staph aureus" agar plates. The overall relative sensitivity was 89%, and the relative specificity was 94%. The sensitivity rose to 100% when excluding low-count subjects (<100 S. aureus colony-forming units per swab).

CONCLUSIONS

This molecular assay is much faster than direct culture and has sensitivity that is appropriate for identification of high-count (>100 S. aureus colony-forming units per swab) nasal S. aureus carriers who are at greatest risk for nosocomial infections.

Cepheid Xpert MRSA cycle threshold in discordant colonization results and as a quantitative measure of nasal colonization burden

We analyzed the cycle threshold (C(T)) of PCR surveillance MRSA swabs obtained from veterans. Lower C(T) on admission was associated with a positive culture from nasal swabs at discharge. Compared to PCR, direct plating of nasal swabs performed poorly, especially for patients with an elevated C(T). The C(T) is strongly correlated with quantitative nasal cultures. Clinical and infection control applications of the C(T) have yet to be defined and warrant further evaluation.

Persistent nasal carriage of Staphylococcus aureus is associated with deficient induction of human beta-defensin 3 after sterile wounding of healthy skin in vivo

Persistent nasal carriage of Staphylococcus aureus is the primary reservoir for this pathogen and a risk factor for infection. The nares of 12 to 30% of healthy individuals are persistently colonized with staphylococci. Elucidating the yet enigmatic determinants of this phenomenon is of major public health interest. We hypothesized that differences in the levels of antimicrobial peptides (AMPs) that are found in human skin and have pronounced antistaphylococcal activity may contribute to this phenomenon. We compared constitutive and induced mRNA levels of RNase 7 and human β-defensin 3 (HBD-3) in healthy and experimentally wounded gluteal skin of 60 volunteers after ascertaining their carrier status through repeated nasal cultures. We found that levels of HBD-3 expression in skin of persistent nasal carriers of S. aureus were lower: induced levels in carriers were 63% (95% confidence interval, 43 to 94%; P = 0.02) and constitutive levels were 76% (95% confidence interval, 52 to 110%; P = 0.14) of those found in noncarriers. No such associations were present for RNase 7. In conjunction with existing knowledge, these findings suggest that healthy individuals with deficient HBD-3 expression in keratinocytes are more prone to persistent nasal colonization with S. aureus.

The role of nasal carriage in Staphylococcus aureus burn wound colonization

Thermal injury destroys the physical skin barrier that normally prevents invasion of microorganisms. This and concomitant depression of local and systemic host cellular and humoral immune responses are important factors that contribute to colonization and infection of the burn wound. One of the most common burn wound pathogens is Staphylococcus aureus. Staphylococcus aureus is both a human commensal and a frequent cause of infections leading to mild to life-threatening diseases. Despite a variety of infection control measures, for example patient cohorting and contact precaution at burn centres, S. aureus is still frequently encountered in burn wounds. Colonization with S. aureus has been associated with delayed wound healing, increased need for surgical interventions, and prolonged length of stay at burn centres. In this minireview, we focus on S. aureus nasal carriage in relation to S. aureus burn wound colonization and subsequent infection, and its impact on strategies for infection control.

Host-microbe interplay in persistent Staphylococcus aureus nasal carriage in HIV patients

It has been shown that persistent Staphylococcus aureus nasal carriage results in increased bacterial dispersal and a higher risk of infection compared to non-or-intermittent S. aureus carriage. Although many studies investigated S. aureus nasal carriage in HIV patients, none compared persistent carriage to non-persistent carriage nor were studies performed in the HAART era. We investigated the host-microbe interplay of persistent S. aureus nasal carriage in HIV-infected patients by studying host determinants of persistent carriage as well as the genetic structure of S. aureus strains isolated. We compared this genetic structure with the previously determined population structure of S. aureus isolates obtained from healthy individuals. Between February 2004 and June 2005 all HIV patients visiting the outpatient department of Erasmus MC (Rotterdam, The Netherlands) were asked to participate in this study. Participants were interviewed and screened for persistent S. aureus carriage using two semi-quantitative nasal swab cultures. For 443 patients two cultures were available, 131 (29.6%) were persistent carriers, which is significantly higher as compared to healthy individuals from the same geographic region (17.6%; P<0.0001). Male sex (odds ratio [OR], 2.22; 95% confidence interval [CI], 1.32-3.73), current smoking (OR, 0.58; 95% CI, 0.38-0.90), Pneumocystis jiroveci pneumonia (PCP) prophylaxis (OR, 0.39; 95% CI, 0.16-0.97) and antiretroviral therapy (OR, 0.61; 95% CI, 0.38-0.98) were independent determinants of persistent carriage. Only two strains were mecA positive (1.2%) and no PVL positive strains were detected. The population structure of S. aureus strains isolated from HIV patients appeared to be strongly overlapping with that of S. aureus isolates from healthy individuals.

Persistent (not intermittent) nasal carriage of Staphylococcus aureus is the determinant of CPD-related infections

BACKGROUND

We investigated the impact of staphylococcal carriage among patients on continuous peritoneal dialysis (CPD) in a university hospital.

METHODS

Patients were screened for Staphylococcus aureus carriage and categorized as persistent, intermittent, or non-S. aureus nasal carriers. Patients were subsequently recultured every 12 weeks for S. aureus and coagulase negative staphylococcal (CoNS) carriage, and followed-up for CPD-related infections and antibiotic resistance.

RESULTS

Fifty-two patients were included: 20 peristent, 10 intermittent, and 22 non-S. aureus carriers. Only persistent S. aureus carriage was significantly associated with an increased risk for all CPD-related infections [incidence rate ratio (IRR) 3.52 (95% CI: 2.56-4.85)], exit site infections [IRR 5.59 (95% CI: 3.50-8.92)], and peritonitis [IRR 2.19 (95% CI: 1.39-3.45)], as well as increased antibiotic use [IRR 3.43 (95% CI: 2.50-4.72)], including vancomycin [IRR 2.15 (95%: 2.13-2.16)]. No vancomycin-resistant S. aureus strains were detected. However, eight (2%) out of 407 CoNS strains isolated were vancomycin intermediately susceptible. In all five patients (four persistent and one intermittent carriers) concerned, this was significantly related to a higher antibiotic (including vancomycin) usage [IRR 2.65 (95% CI: 1.82-3.84)].

CONCLUSION

Persistent-but not intermittent-S. aureus nasal carriage is the major determinant of CPD-related infections, and is associated with a significantly higher consumption of antibiotics, including vancomycin. The highly diverse population of CoNS appears to be the prime reservoir of staphylococcal vancomycin resistance. Accurate determination of the S. aureus nasal carriage state of CPD patients is essential to better target intervention strategies to prevent CPD-related infections.

Staphylococcus aureus nasal carriage in a student community: prevalence, clonal relationships, and risk factors

OBJECTIVE

To evaluate the prevalence and risk factors of nasal Staphylococcus aureus (SA) in the community.

DESIGN

Cross-sectional study.

SETTING

Wake Forest University, Winston-Salem, North Carolina.

PARTICIPANTS

Four hundred fifty students were screened for nasal SA carriage during the fall of 2000, 2001, and 2002.

METHODS

Students were screened by nose swabs. A self-administered questionnaire collected information on demographics and medical history. Antibiotic testing and PFGE were performed on isolates. Risk factors were determined by logistic regression analysis.

RESULTS

Of 450 volunteers, 131 (29%) were SA carriers. Antibiotic resistance was high for azithromycin (26%) and low for ciprofloxacin (1%), tetracycline (5%), mupirocin (1%), and methicillin (2%). PFGE patterns were not associated with carriage. Age, male gender, white race, medical student, allergen injection therapy, chronic sinusitis, rheumatoid arthritis, hospitalization for 6 months or less, and use of antibiotics were associated with carrier status by univariate analysis. Stepwise multivariate logistic regression led to a best fitting model with older age (OR, 1.04; CI95, 1.005-1.079), male gender (OR, 1.50; CI95, 0.982-2.296), and chronic sinusitis (OR, 2.71; CI95, 0.897-8.195) as risk factors. Antibiotic use (< 4 weeks) (OR, 0.41; CI95, 0.152-1.095) and allergen injection therapy (OR, 0.41; CI95, 0.133-1.238) were protective. Analyses of carriers revealed candidate factors for persistent carriage to be nasal SA colonization rate and male gender. Factors for azithromycin resistance were non-medical students and antibiotic use in the past 6 months.

CONCLUSION

Older male volunteers suffering from chronic sinusitis and not taking antibiotics were at higher risk for carrying SA.

Predicting the Staphylococcus aureus Nasal Carrier State: Derivation and Validation of a "Culture Rule"

BACKGROUND

To study determinants and risks of Staphylococcus aureus nasal carriage, adequate differentiation between the different S. aureus carrier states is obligatory. We set out to develop a "culture rule" capable of differentiating between persistent and intermittent or noncarriers that uses a minimum of nasal swab cultures.

METHODS

In 51 healthy volunteers (derivation cohort), 12 quantitative nasal cultures were performed to establish S. aureus nasal carriage states. Persons with 11 or 12 cultures positive for S. aureus were classified as persistent carriers, and those with negative results of all cultures were classified as noncarriers. All other persons were classified as intermittent carriers. By means of logistic regression and receiver operating characteristic (ROC) curves, a culture rule was derived. This culture rule was subsequently validated in 106 participants of an ongoing study in 3882 elderly persons, again with the use of 12 quantitative nasal cultures.

RESULTS

In both cohorts, the positive predictive value of 2 consecutive positive culture results for persistent carriage was 79%. The model best differentiating between persistent and intermittent or noncarriers used the number of positive culture results combined with the amount of S. aureus in these cultures. By using the outcome of 2 cultures, the areas under the ROC curves were 0.981 (95% confidence interval [CI], 0.949-1.0) for the derivation cohort and 0.936 (95% CI, 0.881-0.990) for the validation cohort.

CONCLUSIONS

Combining qualitative and quantitative results of 2 nasal swab cultures accurately predicted the persistent S. aureus carriage state with a reliability of 93.6%. Thus, this culture rule can be used in studies of determinants and risks of S. aureus nasal carriage.

Follow-up of Staphylococcus aureus nasal carriage after 8 years: redefining the persistent carrier state

Studies of Staphylococcus aureus nasal carriage have distinguished three carriage patterns: persistent, intermittent, and noncarriage. The criteria used to identify these carriage patterns have been inconsistent. In 1988 the S. aureus nasal carrier index, i.e., the proportion of nasal swab specimen cultures yielding S. aureus, was determined for 91 staff members of various departments of a large university hospital by obtaining weekly nasal swab specimens for culture over a 12-week period. Thirty-three (36%) persons had carrier indices of 0.80 or higher, 15 (17%) had indices between 0.1 and 0.7, and 43 (47%) had indices of zero. In 1995, 17 individuals with carrier indices of 0.80 or higher in 1988 were available for reexamination. For 12 (71%) of these individuals, S. aureus was again isolated from a single nasal swab, i.e., from each individual with a 1988 carrier index of 1.0 but from only half of those with indices below 1.0. Genotyping (by randomly amplified polymorphic DNA analysis and pulsed-field gel electrophoresis) of all S. aureus strains showed that strains isolated from only three individuals, all with 1988 carrier indices of 1.0, in 1988 and 1995 showed genetic similarity. In conclusion, persistent S. aureus nasal carriage is a unique characteristic of a fraction of the population, and the attribute "persistent" should be confined to those individuals for whom serial nasal swab specimen cultures consistently yield S. aureus.

New approaches to reduce Staphylococcus aureus nosocomial infection rates: treating S. aureus nasal carriage

BACKGROUND

Nosocomial infections cause significant patient morbidity and mortality. The 2.5 million nosocomial infections that occur each year cost the US healthcare system $5 million to $10 million. Staphylococcus aureus has long been recognized as an important pathogen in human disease and is the most common cause of nosocomial infections.

OBJECTIVE

To describe the epidemiology of S. aureus nosocomial infections that are attributable to patients' endogenous colonization.

DATA SOURCES

Review of the English-language literature and a MEDLINE search (as of September 1997).

DATA SYNTHESIS

The ecologic niche of S. aureus is the anterior nares. The prevalence of S. aureus nasal carriage is approximately 20-25%, but varies among different populations, and is influenced by age, underlying illness, race, certain behaviors, and the environment in which the person lives or works. The link between S. aureus nasal carriage and development of subsequent S. aureus infections has been established in patients on hemodialysis, on continuous ambulatory peritoneal dialysis, and those undergoing surgery. S. aureus nasal carriers have a two-to tenfold increased risk of developing S. aureus surgical site or intravenous catheter infections. Thirty percent of 100% of S. aureus infections are due to endogenous flora and infecting strains were genetically identical to nasal strains. Three treatment strategies may eliminate nasal carriage: locally applied antibiotics or disinfectants, systemic antibiotics, and bacterial interference. Among these strategies, locally applied or systemic antibiotics are most commonly used. Nasal ointments or sprays and oral antibiotics have variable efficacy and their use frequently results in antimicrobial resistance among S. aureus strains. Of the commonly used agents, mupirocin (pseudomonic acid) ointment has been shown to be 97% effective in reducing S. aureus nasal carriage. However, resistance occurs when the ointment has been applied for a prolonged period over large surface areas.

CONCLUSIONS

Given the importance of S. aureus nosocomial infections and the increased risk of S. aureus nasal carriage in patients with nosocomial infections, investigators need to study cost-effective strategies to prevent certain types of nosocomial infections or nosocomial infections that occur in specific settings. One potential strategy is to decrease S. aureus nasal carriage among certain patient populations.

Bacterial flora in psoriasis

The aerobic bacterial flora of psoriatic plaques, uninvolved skin and the anterior nares of forty psoriatic patients was studied. The incidence od Staphylococcus aureus was 30% in the anterior nares, 20% on the plaques and 13% on the uninvolved skin. S. aureus counts were 3 x 10(2)/cm2 on the plaques and 1-5 x 10/cm2 on the normal skin. The total bacterial counts were also higher on plaques (7-9 x 10(3)/cm2) than on normal skin (3-0 x 10(3)/cm2). The incidence of lipophilic diphtheroids was significantly lower on the plaques (4%) than the normal skin (30%). Eighty percent of the strains of S. aureus isolated from psoriatic patients were resistant to 10 units of penicillin. Because of increased desquamation, psoriatic skin is a public health hazard.

Effects of a systemic antibiotic on nasal bacterial ecology in man

The nasal flora of coagulase-positive staphylococcus carriers and noncarriers was studied in aerobic conditions in 17 individuals. Five hundred milligrams of cephalexin was given orally four times daily for 12 days, and its effects on the nasal bacteria were determined quantitatively before, during, and after treatment. The total count obtained before the drug treatment was 5.4 x 10(6) in carriers and 3.9 x 10(6) in noncarriers. The lowest total count observed was 3 days after the cessation of the drug. The increase in gram-negative rods was seen 9 days after antibiotic therapy, not during the greatest reduction of gram-positive bacteria. Coagulase-positive cocci and diphtheroids were most sensitive to drug treatment. After 36 days, the total count was restored to pretreatment level. Diphtheroids did not return to the original number and were replaced by a corresponding increase of resistant coagulase-negative cocci. An inverse relationship between coagulase-negative cocci and lipophilic diphtheroids was seen in the anterior nares of many individuals. No gross difference in nasal ecology to differentiate carriers from noncarriers was seen.

Bacteria in a hospital nursery: laboratory and clinical studies

Two hundred and twenty-two infants and fifty-one staff were examined for carriage ofStaphylococcus pyogenesand Gram-negative bacilli, and the presence of these bacteria in the ward environment was also investigated. Staphylococci were phage-typed and tested for sensitivity to antibiotics. Assessment of clinical infection caused by these bacteria was made.

Infant carriage ofStaphylococcus pyogeneswas maximal on the sixth day, 73·1 % of umbilical swabs yielding significant growth. Carriage of Gram-negative bacilli was maximal on the fifth day, 31·7% of umbilical swabs being positive. Where special nursing was enforced, i.e. premature baby unit, isolation of Gram-negative bacilli was very infrequent.

The low nasal carriage of staphylococci seen in infants and nursing staff is attributed to traces of penicillinase-resistant penicillins in the ward air.

Staphylococcus pyogeneswas recovered from 23 % of ward surfaces, and constituted 0·7% of the air-borne flora. In contrast, Gram-negative bacilli were recovered from 4·7 % of surfaces, and composed only 0·15% of the air-borne flora.

Phage group I constituted 73·0, 44·9 and 41·2% of strains from infants, staff and ward environment respectively.

Antibiograms were grouped into three categories, A, B and C. Category A (sensitive to chloramphenicol, erythromycin and methicillin but penicillin-resistant) composed 79·0% of infants' strains, being found mostly in group I, but was rare among staff strains.

Of 54 specimens taken from 40 infants showing evidence of infection,Staphylococcus pyogeneswas isolated from 16, of which 14 were category A. Gram-negative bacilli were isolated 7 times. Only 21 infants required antibiotic therapy (9·3%) and none were infected with Gram-negative bacilli—this contrasts with the frequency of these bacilli in nursery infection today.

Ward air is considered to be the maindepotofStaphylococcus pyogenes. Infant ‘dispersers’ are probably the principalsourceof the air-borne staphylococci. Infants and staff abstract from the air those staphylococci which become their colonizing strains. No transfer is thought to occur from the staff to the infants. Transmission of Gram-negative bacilli is similar in that the infants are the principal source and the air the maindepotof these bacteria. Nasal colonization by Gram-negative bacilli is low, this is possibly due to poor colonizing properties. Infant umbilical carriage almost certainly results from endogenous faecal spread.

We are grateful to Dr J. C. Gould, Director, Central Microbiological Laboratories, Edinburgh, for his continued interest and advice, and to Mr J. C. Ferguson and Mrs M. M. Winton for skilled technical assistance. This study would not have been completed without the co-operation of Sisters McCartney and Struthers and their nursing staff.