rosserial, publish a ROS topic with uint16_t and strings

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I'm using a Arduino MEGA 2560 to read from different spectrometers and then I need to publish a ROS topic for each sensor. I'm on ROS Melodic.

The problem is that I read each sensor channel with:

as7341_0.getChannel(AS7341_CHANNEL_415nm_F1)
as7341_0.getChannel(AS7341_CHANNEL_415nm_F1

and so on, and getChannel is defined in this way

uint16_t Adafruit_AS7341::getChannel    (   as7341_color_channel_t  channel )

I publish the ROS topic by using this string:

std_msgs::String str_msg;

How can I concatenate all the getChannel values and copy them to str_msg? Is it possible?

I would like to publish something like this with commas:

result_from_getChannel1,result_from_getChannel2,result_from_getChannel3

Thank you!