Ultrasonic imaging of biofilms utilizing echoes from the biofilm/air interface

Detection and quantification of bacterial biofilms combining high-frequency acoustic microscopy and targeted lipid microparticles

BACKGROUND

Immuno-compromised patients such as those undergoing cancer chemotherapy are susceptible to bacterial infections leading to biofilm matrix formation. This surrounding biofilm matrix acts as a diffusion barrier that binds up antibiotics and antibodies, promoting resistance to treatment. Developing non-invasive imaging methods that detect biofilm matrix in the clinic are needed. The use of ultrasound in conjunction with targeted ultrasound contrast agents (UCAs) may provide detection of early stage biofilm matrix formation and facilitate optimal treatment.

RESULTS

Ligand-targeted UCAs were investigated as a novel method for pre-clinical non-invasive molecular imaging of early and late stage biofilms. These agents were used to target, image and detect Staphylococcus aureus biofilm matrix in vitro. Binding efficacy was assessed on biofilm matrices with respect to their increasing biomass ranging from 3.126 × 103 ± 427 UCAs per mm(2) of biofilm surface area within 12 h to 21.985 × 103 ± 855 per mm(2) of biofilm matrix surface area at 96 h. High-frequency acoustic microscopy was used to ultrasonically detect targeted UCAs bound to a biofilm matrix and to assess biofilm matrix mechanoelastic physical properties. Acoustic impedance data demonstrated that biofilm matrices exhibit impedance values (1.9 MRayl) close to human tissue (1.35 - 1.85 MRayl for soft tissues). Moreover, the acoustic signature of mature biofilm matrices were evaluated in terms of integrated backscatter (0.0278 - 0.0848 mm(-1) × sr(-1)) and acoustic attenuation (3.9 Np/mm for bound UCAs; 6.58 Np/mm for biofilm alone).

CONCLUSIONS

Early diagnosis of biofilm matrix formation is a challenge in treating cancer patients with infection-associated biofilms. We report for the first time a combined optical and acoustic evaluation of infectious biofilm matrices. We demonstrate that acoustic impedance of biofilms is similar to the impedance of human tissues, making in vivo imaging and detection of biofilm matrices difficult. The combination of ultrasound and targeted UCAs can be used to enhance biofilm imaging and early detection. Our findings suggest that the combination of targeted UCAs and ultrasound is a novel molecular imaging technique for the detection of biofilms. We show that high-frequency acoustic microscopy provides sufficient spatial resolution for quantification of biofilm mechanoelastic properties.

The use of ultrasonic imaging to evaluate the effect of protozoan grazing and movement on the topography of bacterial biofilms

AIMS

This study evaluated the effect of protozoan movement and grazing on the topography of a dual-bacterial biofilm using both conventional light microscopy and a new ultrasonic technique.

METHODS AND RESULTS

Coupons of dialysis membrane were incubated in Chalkley's medium for 3 days at 23 degrees C in the presence of bacteria (Pseudomonas aeruginosa and Klebsiella aerogenes) alone, or in co-culture with the flagellate Bodo designis, the ciliate Tetrahymena pyriformis or the amoeba Acanthamoeba castellanii. Amoebic presence resulted in a confluent biofilm similar to the bacteria-only biofilm while the flagellate and ciliate created more diverse biofilm topographies comprising bacterial microcolonies and cavities.

CONCLUSIONS

The four distinct biofilm topographies were successfully discerned with ultrasonic imaging and the method yielded information similar to that obtained with conventional light microscopy.

SIGNIFICANCE AND IMPACT OF THE STUDY

Ultrasonic imaging provides a potential way forward in the development of a portable, nondestructive technique for profiling the topography of biofilms in situ, which might aid in the future management of biofouling.

High-accuracy data acquisition architectures for ultrasonic imaging

This paper proposes a novel architecture for a data acquisition system intended to support the next generation of ultrasonic imaging instruments operating at or above 100 MHz. Existing systems have relatively poor signal-to-noise ratios and are limited in terms of their maximum data sampling rate, both of which are improved by a combination of embedded averaging and embedded interleaved sampling. "On-the-fly" pipelined operation minimizes control overheads for signal averaging. A two-clock sampling timing system provides for effective sampling rates that are a factor of 20 or more above the basic sampling rate of the analog-to-digital converter (ADC). The system uses commercial field-programmable gate array devices operated at clock frequencies commensurable with the ADC clock. Implementation is via the Xilinx Xtreme digital signal processing development kit, available at low cost. Sample rates of up to 2160 MHz have been achieved in combination with up to 16384 coherent averages using the above-mentioned off-the-shelf hardware.

High frequency ultrasound imaging of a single-species biofilm

OBJECTIVE

This study evaluated the feasibility of a high frequency ultrasound scan to examine the 3D morphology of Streptococcus mutans biofilms grown in vitro.

METHODS

Six 2-day S. mutans biofilms and six 7-day biofilms were grown on tissue culture membranes and on bovine dentine discs. A sterile growth medium on the membrane and disc were used as controls. Surfaces were rinsed and then immersed in sterile saline. High-frequency ultrasound imaging system was used to scan these surfaces at 55MHz, and a computer program calculated the average thickness of the biofilm layer from the 3D images.

RESULTS

3D pictures of the biofilm layers were obtained. Different cross-sections and plains are easily demonstrated. The average thickness of the 7-day biofilm was significantly bigger than the 2-day on both the membranes and dentinal discs. No structures were observed on the sterile membrane or disc.

CONCLUSION

Three-dimensional structural imaging in situ is possible without damaging the biofilm layer in a quick and easy manner and can therefore be used to evaluate biofilms longitudinally as a function of time.