Paired methods to measure biofilm killing and removal: a case study with Penicillin G treatment of Staphylococcus aureus biofilm

Biofilms are microbial aggregates that show high tolerance to antibiotic treatments in vitro and in vivo. Killing and removal are both important in biofilm control, therefore methods that measure these two mechanisms were evaluated in a parallel experimental design. Kill was measured using the single tube method (ASTM method E2871) and removal was determined by video microscopy and image analysis using a new treatment flow cell. The advantage of the parallel test design is that both methods used biofilm covered coupons harvested from a CDC biofilm reactor, a well-established and standardized biofilm growth method. The control Staphylococcus aureus biofilms treated with growth medium increased by 0·6 logs during a 3-h contact time. Efficacy testing showed biofilms exposed to 400 μmol l^-1 penicillin G decreased by only 0·3 logs. Interestingly, time-lapse confocal scanning laser microscopy revealed that penicillin G treatment dispersed the biofilm despite being an ineffective killing agent. In addition, no biofilm removal was detected when assays were performed in 96-well plates. These results illustrate that biofilm behaviour and impact of treatments can vary substantially when assayed by different methods. Measuring both killing and removal with well-characterized methods will be crucial for the discovery of new anti-biofilm strategies.

SIGNIFICANCE AND IMPACT OF THE STUDY

Biofilms are tolerant to antimicrobial treatments and can lead to persistent infections. Finding new anti-biofilm strategies and understanding their mode-of-action is therefore of high importance. Historically, antimicrobial testing has focused on measuring the decrease in viability. While kill data are undeniably important, measuring biofilm disruption provides equally useful information. Starting with biofilm grown in the same reactor, we paired assessment of biofilm removal using a new treatment-flow-cell and real-time microscopy with kill data collected using the single tube method (ASTM E2871). Pairing these two methods revealed efficient biofilm removal properties of Penicillin G which were not detected during efficacy testing.

Diffusive transport through a model host-biofilm system

Free-living biofilms have been subject to considerable attention, and basic physical principles for them are generally accepted. Many host-biofilm systems, however, consist of heterogeneous mixtures of aggregates of microbes intermixed with host material and are much less studied. Here we analyze a key property, namely reactive depletion, in such systems and argue that two regimes are possible: (1) a homogenizable mixture of biofilm and host that in important ways acts effectively like a homogeneous macrobiofilm and (2) a distribution of separated microbiofilms within the host with independent local microenvironments.

A repeatable laboratory method for testing the efficacy of biocides against toilet bowl biofilms

AIMS

The purpose of this study was to develop a laboratory biofilm growth reactor system that simulated the toilet bowl environment and which could be used for biocide efficacy testing.

METHODS AND RESULTS

A microbial biofilm reactor system incorporating intermittent flow and nutrient provision was designed. The reactor system was open to the air and was inoculated with organisms collected from toilet bowl biofilms. Once per hour, reactors were supplied with a nutrient solution for a period of 5 min, then flushed and refilled with tap water or tap water amended with chlorine. Quantitative measures of the rate and extent of biofilm accumulation were defined. Biofilm accumulated in untreated reactors to cell densities of 108 cfu cm-2 after approximately 1 week. Biofilm accumulation was also observed in reactors in the continuous presence of several milligrams per litre of free chlorine. Repeatability standard deviations for the selected efficacy measures were low, indicating high repeatability between experiments. Log reduction values of viable cell numbers were within ranges observed with standard suspension and hard surface disinfection tests. Biofilm accumulated in laboratory reactors approximately seven times faster than it did in actual toilet bowls. The same ranking was achieved in tests between laboratory biofilms and field-grown biofilms with three of the four measures, using three different concentrations of chlorine.

CONCLUSION

This reactor system has been shown to simulate, in a repeatable way, the accumulation of bacterial biofilm that occurs in toilet bowls. The results demonstrate that this system can provide repeatable assays of the efficacy of chlorine against those biofilms.

SIGNIFICANCE AND IMPACT OF THE STUDY

The laboratory biofilm reactor system described herein can be used to evaluate potential antimicrobial and antifouling treatments for control of biofilm formation in toilet bowls.

Color measurement as a means of quantifying surface biofouling

Laboratory reactors fitted with removable ceramic porcelain growth surfaces were inoculated with a consortium of biofilm forming environmental isolates. A Minolta colorimeter CR-200 (Minolta Camera Co., Ltd, Ramsey, NJ) was used in conjunction with a specially designed adapter to evaluate the reflective color of the porcelain disks as biofilm accumulated on them. Areal viable cell counts were monitored over a period of eleven days in two separate experiments and direct color measurements of the untreated, microbially fouled test surfaces were collected. This colorimetric assay was both non-destructive and immediate. A strong linear relationship between log cell density and log color change was observed. The Pearson product moment correlation coefficient for all 45 observations combined was r = 0.95. Separate regression lines for each experiment were not significantly different (P = 0.19). When adjusted for time, the (partial) correlation coefficient between log cell density and log color change was r = 0.87, which suggests that the relationship between the two measures can not be explained by their mutual dependence on time. Reflective color measurement provided a rapid, non-destructive and quantitative measure of biofllm accumulation.

Effects of selective inhibitors on cyclic nucleotide phosphodiesterases of rabbit aorta

In this study three forms of cyclic nucleotide phosphodiesterase (PDE) isolated from rabbit aorta were pharmacologically characterized, and the consequence of selective inhibition of calmodulin-stimulated PDE (CaM-PDE) and cGMP specific PDE (cG-PDE) was evaluated using PDE inhibitors. The cG-PDE (F1) was selectively inhibited by M&B 22948 (IC50 = 0.5 microM) and dipyridamole (IC50 = 7 microM). The cAMP-PDE (cA-PDE, F3) was inhibited more effectively by the cA-PDE inhibitor milrinone than by other PDE inhibitors. The cA-PDE preparation appeared to contain both cG-inhibited PDE and cG-insensitive PDE based on an additive inhibition of the activity by milrinone and SQ 65442, respective inhibitors of these enzymes. Vinpocetine, 8-methoxymethyl isobutylmethylxanthine (8-MeOMeMIX) and M&B 22948 effectively inhibited CaM-PDE (F2). Vinpocetine was a more selective inhibitor of CaM-PDE than M&B 22948 or 8-MeOMeMIX. CaM-PDEs isolated from rabbit aorta and bovine brain exhibited a similar sensitivity to these inhibitors. Seventy-two percent of the cGMP-hydrolyzing activity of this rabbit aortic CaM-PDE preparation was immunoadsorbed to monoclonal antibody (ACC-1) against CaM bound to brain CaM-PDE. Vinpocetine, 8-MeOMeMIX and M&B 22948 at concentrations (30 and 100 microM) which inhibit CaM-PDE greater than 60% increased cGMP but not cAMP levels in l-norepinephrine (NE) preincubated rabbit aortic slices. At concentrations selectively inhibiting cG-PDE, dipyridamole and M&B 22948 increased cGMP levels in untreated slices but failed to increase cGMP levels significantly in NE-treated slices. By contrast, vinpocetine failed to increase cGMP significantly in untreated slices, although it increased cGMP levels in NE or KCl preincubated slices. These data indicate that, in activated (precontracted) aorta, CaM-PDE is a major enzyme, whereas in untreated aorta cG-PDE is a predominant enzyme for the hydrolysis of cGMP. This study also shows a usefulness of selective inhibitors in identifying different forms of PDE and similar drug sensitivities and immunoadsorption of aortic and brain CaM-PDEs by a monoclonal antibody.

SCH 39370, a neutral metalloendopeptidase inhibitor, potentiates biological responses to atrial natriuretic factor and lowers blood pressure in desoxycorticosterone acetate-sodium hypertensive rats

SCH 39370 (N-[N-[1-(S)-carboxyl-3-phenylpropyl]-(S)-phenyl-alanyl]-(S)-isoserin e) is a potent and specific inhibitor of neutral metalloendopeptidase (NEP) from rabbit kidney (IC50 = 11.2 +/- 1.9 nM) and is devoid of angiotensin-converting enzyme inhibitory activity at 1 microM. We evaluated the effect of NEP inhibition with SCH 39370 on the inactivation of atrial natriuretic factor (ANF) and on cardiovascular function in rats. SCH 39370 effectively prevented in vitro degradation of ANF (99-126) by a purified rabbit kidney NEP. SCH 39370 (30 mg/kg s.c) significantly delayed the disappearance of immunoreactive (ir) ANF from plasma in rats after an i.v. infusion of ANF (1 microgram/kg/min for 30 min): the plasma ir ANF level at 15 min postinfusion was 1.5 +/- 0.3 ng/ml vs. 0.3 +/- 0.04 ng/ml in the control. SCH 39370 also delayed the disappearance of ir ANF after infusion of the peptide (0.1 microgram/kg/min for 30 min) which increased plasma levels to those observed during volume expansion. This effect was accentuated markedly in rats with bilateral nephrectomy. The hypotensive response to injection of ANF (30 micrograms/kg i.v.) in spontaneously hypertensive rats (-38 +/- 6 mm Hg vs. -13 +/- 2 mm Hg in the control) and the diuretic and natriuretic responses to ANF in normal rats were potentiated by SCH 39370 (30 mg/kg s.c.), respectively. The results suggest that NEP can play a role in ANF disposition in vivo and that potentiation of the biological activities of high doses of ANF by SCH 39370 may be consequent to its inhibitory effect on ANF degradation.(ABSTRACT TRUNCATED AT 250 WORDS)